-
ORIGINAL PAPER
The basal dunite of the Precambrian mafic-ultramafic
Näränkävaaraintrusion: Petrogenetic considerationsand implications
to exploration
Ville Järvinen1 & Tapio Halkoaho2 & Jukka Konnunaho3
& Jussi S. Heinonen1 & O. Tapani Rämö1
Received: 17 March 2020 /Accepted: 16 September 2020# The
Author(s) 2020
AbstractSeveral mafic-ultramafic layered intrusions were
emplaced in the Fennoscandian Shield during wide-spread
mantle-sourcedmagmatism at 2.5–2.4 Ga. The Näränkävaara intrusion
(surface area 5 × 30 km2), northeastern Finland, contains a 1.5–2
km thickbasal dunite (not dated), and a 1.5 km thick layered series
(2436 ± 5 Ma). A newly discovered marginal series between the
basaldunite and the layered series indicates that the basal dunite
is older, and highlights the need for further study on their
relationship.Along its southern basement contact, the basal dunite
contains a 200–300 m thick zone of olivine ortho- and
mesocumulates, butthe bulk of it is composed of olivine adcumulates
and lesser olivine-orthopyroxene heteradcumulates. Based on
whole-rockgeochemistry, the basal dunite is divided into a low-Fe
zone (average FeOt 10.2 wt% and Ni 2250 ppm) and a high-Fe
zone(average FeOt 12.5 wt% and Ni 1700 ppm). Both zones have high
MgO (32–47 wt%) and varying Cr (830–5160 ppm) andAl2O3/TiO2
(16–26). Textural and geochemical layering is similar along the 30
km strike of the basal dunite. A LREE-enrichedhigh-MgO basaltic
parental magma composition (13–18 wt% MgO) is inferred for the
basal dunite from olivine–melt mixingtrends in orthocumulates. The
dunite exhibits at least two geochemical reversals as well as
abundant low-porosity adcumulates,poikilitic chromite, and bimodal
olivine, suggesting formation in a high-volume open magmatic
system. Significant similarity inmajor and trace element
compositions with the Näränkävaara layered series and the
Burakovsky intrusion and Vetreny beltextrusives in RussianKarelia
suggests that the basal dunite belongs to the Fennoscandian 2.5–2.4
Gamafic layered intrusions. AnArchean komatiitic origin for the
dunite body cannot be completely ruled out, however. Distinct
Ni-depletion in olivine is foundin the basal dunite from the low-Fe
zone to the high-Fe zone (3200 versus 2200 ppm). This depletion
does not correlate with Focontents, which suggests that it is not
related to olivine fractionation. The basal dunite may thus have
potential for Ni-(Cu-Co-PGE) sulfide mineralization.
Keywords Tonio-Näränkävaara belt . Näränkävaara . Layered
intrusion . Dunite . Komatiite . Ni-criticality
Introduction
The Tornio-Näränkävaara intrusive belt (TNB) in north-ern
Finland is critical for orthomagmatic PGE-Ni-Cu-Coand Cr-V-Ti-Fe
deposits (Huhtelin 2015; Iljina et al.2015; Makkonen et al. 2017).
The easternmost of these2.44 Ga layered intrusions (Iljina and
Hanski 2005) isthe mafic-ultramafic Näränkävaara intrusion (Fig.
1).The geology and mineral potential of the Näränkävaaraintrusion
is still relatively poorly known (Alapieti 1982;Järvinen et al.
2020). The presence of a large mass ofultramafic cumulates below
the layered series of theNäränkävaara intrusion (Fig. 2a) has been
known sincethe 1960’s (Auranen 1969), but the petrogenesis,
miner-al potential, and relationship of this basal dunite with
Editorial handling: L. Nasdala
Electronic supplementary material The online version of this
article(https://doi.org/10.1007/s00710-020-00725-9) contains
supplementarymaterial, which is available to authorized users.
* Ville Jä[email protected]
1 Department of Geosciences and Geography, University of
Helsinki,Gustaf Hällströmin katu 2 (Physicum), P.O. Box 64,00014
Helsinki, Finland
2 Geological Survey of Finland, Neulaniementie 5, P.O. Box
1237,70211 Kuopio, Finland
3 Geological Survey of Finland, Lähteentie 2, P.O. Box 77,96101
Rovaniemi, Finland
https://doi.org/10.1007/s00710-020-00725-9
/ Published online: 10 November 2020
Mineralogy and Petrology (2021) 115:37–61
http://crossmark.crossref.org/dialog/?doi=10.1007/s00710-020-00725-9&domain=pdfhttp://orcid.org/0000-0002-1907-1680https://doi.org/10.1007/s00710-020-00725-9mailto:[email protected]
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the neighboring layered series have not been resolved.More
recent mineral exploration projects have reportedthe basal dunite
to be composed of homogeneous,~2 km thick olivine adcumulate, and
to be continuousfor the 30 km length of the intrusion (Fig. 2a)
(Iljina2003; Vesanto 2003; Lahtinen 2005). The basal duniteexhibits
several features commonly thought of askomatiitic, being mostly
composed of tightly packedmonomineralic olivine adcumulate,
sometimes with bi-modal olivine and poikilitic chromite (cf. Barnes
andHill 1995). The recent discovery of a marginal seriesbetween the
Näränkävaara layered series and the basaldunite indicates that the
basal dunite must be at leastmarginally older (Järvinen et al.
2020).
Here, the Näränkävaara basal dunite is hypothesized torepresent
either a feeder channel cumulate related to thesame Fennoscandian
2.5–2.4 Ga magmatism as the
2436 Ma Näränkävaara layered series (Fig. 2a) (Alapieti1982;
Kulikov et al. 2010), or an older komatiitic wall-rock for the
layered ser ie s ex tending in to theNäränkävaara area from the
nearby Archean Takanen orSuomussalmi greenstone belts (Iljina 2003;
Papunen et al.2009; Mikkola et al. 2011). Mantle-derived ultramafic
cu-mulates are known to host orthomagmatic sulfide deposits(e.g.
Begg et al. 2010; Barnes et al. 2016a); in the formercase it could
have potential for magmatic sulfide mineral-izations linked to
feeder channel cumulates, such as inJinchuan (Chai and Naldrett
1992), and in the latter casefor komatiite-hosted sulfide
mineralizations (Konnunaho2016; Makkonen et al. 2017).
New drilling and mapping was conducted in theNäränkävaara basal
dunite during 2017–2018 by theGeological Survey of Finland (GTK)
(Halkoaho et al. 2019).Based on these new results and a compilation
of pre-existing
Fig. 1 Map of 2.53–2.39 Ga mafic-ultramafic layered intrusions
of theFennoscandian Shield. The Tornio-Näränkävaara belt is
outlined with ared dashed line, and a white arrow points out
Näränkävaara. TheKoillismaa complex includes the tectonically
dismembered western
intrusions and Näränkävaara to the east, connected by a positive
magneticand gravity anomaly presumed to be a “hidden dyke” (thin
black dashedline) (map modified after Alapieti 1982)
38 V. Järvinen et al.
-
data (Alapieti 1982; Iljina 2003; Vesanto 2003; Lahtinen2005;
Akkerman 2008; Telenvuo 2017; Järvinen et al.2020), we characterize
the Näränkävaara basal dunite, modelits petrogenesis, and estimate
its Ni-(Cu-Co-PGE) potential.
Geological background for the 2.5–2.4 Gamagmatism in the
Fennoscandian shieldand the Tornio-Näränkävaara belt in Finland
The northern part of the Fennoscandian shield (mainly north-ern
Finland and NW Russia) hosts 20–40 mafic-ultramaficlayered
intrusions and associated dyke swarms with ages be-tween 2.53–2.39
Ga (Fig. 1) (Alapieti et al. 1990; Amelinet al. 1995; Iljina and
Hanski 2005; Bayanova et al. 2009).These intrusions are located
mainly along rif ted
intracontinental paleoplate boundaries in the ArcheanKarelia and
Kola cratons (Bayanova et al. 2009; Kulikovet al. 2010; Tiira et
al. 2014).
The five intrusions and intrusion complexes comprising theTNB
have an average age of 2.44 Ga (Fig. 1) (Iljina andHanski 2005).
They are typical layered intrusions with a lay-ered and marginal
series, and are divided into three composi-tional groups: (1)
mafic-ultramafic, which contain aperidotitic–pyroxenitic lower zone
(Näränkävaara andKemi), (2) mafic, which are dominated by
gabbronoriticrocks (Koillismaa), and (3) megacyclic (Penikat
andPortimo), which are characterized by macrorhythmiccompositional
reversals (Alapieti and Lahtinen 2002).Stratigraphically, the
intrusions of the TNB are foundat the contacts of late Archean
granitoids and overlyingPaleoproterozoic supracrustal rocks
(Alapieti et al.
R10
R7
R8
R5R4
X’
?
?
??
?
?
?
?
High-Fe subzoneLow-Fe subzone
High-Fe subzone
Low-Fe subzone
BASAL DUNITE
LOW-FE ZONE HIGH-FE ZONE
HeteradcumulateAdcumulate
Adcumulate (low
-Mg#)
Adcumulate
(Ol)-O
rthopyroxeniteAdcumulate
Orthocum
ulate
Meso -- Adcum
ulate
High Ni
Variable Chr
Lobate Chr
Euhedral Chr
Variable Chr
Poikilitic Chr Lobate Chr
Euhedral Chr
Euhedral Chr
Harzburgite and Pyroxenite
??
?
MARGINALSERIES
Moderate Ni
R8 R5 R4 R2 R1
Low-Ni
R9
Hei
ght (
m)
300
0
-100
ADCUMULATE & HETERADCUMULATEORTHO- & MESOCUMULATE
(BORDER ZONE)
Granite-gneiss
Plagioclase-websterite
LAYEREDSERIES
200
100
X X’
0 m 250 m 500 m
R3
R6Amphibolite
2 km
X
R1
R2
Granite-gneiss
BASAL DUNITE
LAYERED SERIES
Harzburgite, pyroxenite
Gabbronorite, gabbro
Plagioclase websteriteMARGINAL SERIES
Melagabbronorite
HIGH-FE ZONE
OrthopyroxeniteLOW-FE ZONE
High-NiOlivine ortho–mesocumulate
AmphiboliteARCHEAN BASEMENT
DiabasePROTEROZOIC DIKES
Granite-gneiss
Olivine ortho–mesocumulateModerate Ni
Olivine adcumulateLow Mg#
5 km
Takanen
FINSuomussalmi
RUSNäränkävaara
N
Hand-sampleDrill-hole and trace
1
2
Olivine adcumulateHigh-Ni3
Olivine adcumulateModerate-Ni4
5
Olivine adcumulate6
7
Olivine adcumulate8
10
9
1 23
45
6 78
9
10
1
2 3 5 6 7 8 9 10
c
b
a
2
Olivine-orthopyroxeneheteradcumulate
Russia
Poikilitic harzburgite
2.44 Ga Layered series, mafic zone
2.44 Ga Layered series, ultramafic zone
Archean greenstone belts
Basal dunite, Low-Fe zone
Basal dunite, Hi-Fe zone
Northern dunite
Granite-gneissbasement complex
NW block cross sectionfrom Järvinen et al. 2020
Fig. 2 a Simplified geological map of the Näränkävaara
intrusion. Theintrusion continues about 2 km into Russia; b
Geological map of the SEblock of the Näränkävaara basal dunite with
locations of new drill holes(R1-R10) and surface samples (white
triangles); c Cross-section X-X’along the Näränkävaara basal
dunite, with the SW basement contact to
the left. Contacts are mostly unexposed and extrapolated from
knowncontacts or inferred from geophysical measurements. Note that
most geo-chemical diagrams in this paper are plotted with the same
unit colors asare used here
39The basal dunite of the Precambrian mafic-ultramafic
Näränkävaara intrusion: Petrogenetic considerations...
-
1990). In contrast to other intrusions of the TNB,
theNäränkävaara intrusion is completely surrounded by theArchean
granite-gneiss basement complex (Fig. 1) (Hölttäet al. 2012). In
addition, Näränkävaara is the only intrusionin the TNB associated
with voluminous dunitic cumulates(Alapieti 1982; Järvinen et al.
2020).
The parental magmas of the Finnish 2.5–2.4 Gamafic
layeredintrusions are thought to be (boninite-like) siliceous
high-Mgbasalts (Alapieti et al. 1990; Iljina and Hanski 2005).
Whole-rock and isotope geochemical systematics (average initial
εNdof −2) point to crustal contamination of primitive magmas froma
mantle plume source (Puchtel et al. 1997; Hanski et al. 2001;Vuollo
and Huhma 2005; Yang et al. 2016; Rämö et al. 2017;Maier et al.
2018). Intrusive and extrusive formations with sim-ilar
compositions are found in Russian Karelia, for example
thekomatiitic basalts and dykes of the Vetreny belt, and the
mafic-ultramafic Burakovsky layered intrusion (Fig. 1) (Puchtel et
al.1997; Chistyakov and Sharkov 2008).
The Näränkävaara layered intrusion has been included as apart of
the Koillismaa layered intrusion complex (KLIC)(Alapieti 1982). The
KLIC comprises (1) the tectonically dis-membered western intrusion
blocks of the Koillismaa layeredintrusion (Karinen 2010), (2) the
eastern Näränkävaara intrusion,and (3) a strong positive gravity
and magnetic anomalyconnecting the first two (dashed line in Fig.
1). Alapieti (1982)interpreted the Koillismaa and Näränkävaara
intrusions to be co-genetic, with the Näränkävaara intrusion
representing the lowestexposed stratigraphic level of the KLIC. The
connecting geo-physical anomaly – hitherto not intersected in
drilling – wasthought to be a magmatic conduit that fed the two
intrusions.The recently discovered (likely Archean) Takanen schist
belt(Fig. 2a) coincides with the geophysical anomaly (Iljina
2003;Salmirinne and Iljina 2003; Iljina et al. 2006).
Geology of the Näränkävaara intrusion
The mafic-ultramafic Näränkävaara intrusion (Fig. 2a) ismostly
unexposed but shows as a strong positive magneticanomaly about 30
km long and 5 km wide, with the basaldunite also being highly and
untypically conductive(Niskanen and Jokinen 2018). Gravimetric and
magnetomet-ric measurements indicate that the intrusion extends to
a depthof 5–10 km (Elo 1992; Salmirinne and Iljina 2003). A
largeSW–NE trending fault separates the Näränkävaara intrusioninto
two blocks (NW and SE block).
The intrusion comprises the following three distinct igne-ous
bodies (or series) that are partly or dominantly composedof
ultramafic cumulates (Fig. 2a): (1) The southernmost unitconsists
of a 1–2 km thick basal dunite (not dated thus far) thatis composed
of dunite and minor peridotite and pyroxenite(Vesanto 2003;
Järvinen et al. 2020). This unit is in the focusof the present
study. (2) The central, 2436 ± 5 Ma (2σ) old
Näränkävaara layered series is composed of a 700 m
thickperidotitic–pyroxenitic ultramafic zone, and a 600
mgabbroic–dioritic mafic zone (Alapieti 1982). In the SE blockof
the intrusion, a poorly developed 10 m thick gabbronoriticmarginal
series was recently found in the contact between thebasal dunite
and the layered series, grading into the layeredseries (Järvinen et
al. 2020). (3) A series of smaller elongateintrusions runs parallel
to the northern contact of the layeredseries and composed of
poikilitic harzburgite (Alapieti et al.1979). These northern
harzburgites have been drilled (Iljina2003) but have not been
investigated in detail.
In the layered series, the dip of igneous layering in the
NWblock is 20 ° to the NE, while in the SE block it is 5–15 ° to
theSW (Fig. 2a) (Alapieti et al. 1979). In addition to the
largefault splitting the intrusion, there are several strike-slip
faultsalong the short axis of the body, and dip-slip faults with
strikesalong the long axis. Apart from faulting the intrusion is
mostlyundeformed. The layered series is relatively unaltered,
where-as the basal dunite is thoroughly serpentinized.
One previous drilling profile located in the NW block ofthe
intrusion intersects the basal dunite, but includes severalgaps up
to 600 m in thickness. Järvinen et al. (2020) dividedthe basal
dunite along this NW cross section into three olivinecumulate units
(see Fig. S1 in electronic supplementarymaterial), from south to
north: (1) a harzburgitic olivinemesocumulate transition zone in
contact with the basementcomplex and characterized by distinctly
higher contents ofincompatible elements compared to the other two
units, (2) ahomogeneous olivine adcumulate center section
characterizedby relatively low Cr contents (1000–1500 ppm Cr)
comparedto the other two units (2000–4000 ppm Cr), and (3) an
olivinead- to mesocumulate unit characterized by coarse
poikiliticorthopyroxene, in contact with the layered
series.Orientation of igneous layering in the NW block of the
basaldunite is poorly constrained, but is assumed to be
subverticalat the southern basement contact and to get shallower
towardsthe north, being about 45 ° at the basal dunite–layered
seriescontact. No gabbronoritic marginal series is found at this
con-tact in the NW block, unlike in the SE block.
In addition to the previously mentioned cumulate units,600 m of
homogeneous olivine adcumulate was intersectedin one drill hole on
the northern side of the layered series in theSE block of the
intrusion (indicated as ‘northern dunite’ inFig. 2a) (Akkerman
2008). This northern dunite has a muchlower Al2O3/TiO2 ratio than
the main basal dunite body onsouth side of the layered series
(average of 7 versus 21), andhas not been found elsewhere.
Materials and methods
The materials for this study were obtained during new
drillingand mapping activities by the Geological Survey of Finland
in
40 V. Järvinen et al.
-
2017–2018 (Halkoaho et al. 2019). New drilling and sampling
islocated in the SE block of the Näränkävaara intrusion, and
com-prises 10 drill holes with 2380 m of drill core and 109
analyzedwhole-rock samples (drill holes R1-R10 in Fig. 2b), and 18
sur-face samples. Pre-existing data has been taken into
considerationfor purposes of petrographical and geochemical
correlations andinterpretations, but has generally not been plotted
in diagrams forsake of clarity. All sample materials used, and
sources used fordata compilation, are listed in Table S1.
Full whole-rock analysis results, method descriptions,
detec-tion limits, and calibration materials used are listed in
Table S2.Explanations of method codes and method descriptions
canalso be found in the reference Labtium Oy (2015). Major
andsomeminor elements were determined with a Panalytical AxiosPW
4400 X-ray fluorescence (XRF) spectrometer from pressedpowder
pellets (Labtium Oy method code 176X). Platinum, Pdand Au were
determined by Pb-fire assay fusion followed bygraphite furnace
atomic absorption spectrometry (GFAAS;method code 704 U). For the
rare-earth elements (REE) andsome trace elements, sample solutions
were prepared by HF–HClO4 digestion and lithium metaborate–sodium
perborate fu-sion, followed by inductively coupled plasma mass
spectrom-etry (ICP-MS) with a Thermo Electron iCAP Qc (method308
M), or optical emission spectrometry (ICP-OES) with aThermo
Electron iCAP 6500 (method 308P), with final resultscalculated as
an average of three replicate analyses. For allanalysis methods,
duplicate analyses were made for 5% ofsamples, with a coefficient
of variation generally below 5%.All results of whole-rock analyses
presented in this study arenormalized to volatile-free 100 wt%.
For parental magma modeling, and for calculating whole-rock Mg#
[Mg# =Mg/(Mg + Fe2+)], whole-rock Fe2O3/FeOratios have been
calculated from total FeO (FeOt) accordingto the method of Barnes
et al. (2007). This method was devel-oped for pure olivine
adcumulates (komatiites) where Fe2O3contents is assumed to be zero,
as Fe3+ is incompatible inolivine. A linear increase in whole-rock
Fe2O3/FeO ratio from0 to 0.1 is calculated as a function of
decreasing modal olivineand increasing liquid component. Olivine
mode is proxiedusing whole-rock MgO contents, so that a Fe2O3/FeO
ratioof 0.1 results at 50 wt%MgO, with a constant linear variation
in the ratio in between(Barnes et al. 2007).
Petrography of 208 thin sections were examined with
cross-polarised transmitted-light microscopy. Chemical
compositionsof minerals from the SE block of the Näränkävaara body
wereanalyzed bywavelength dispersive spectroscopywith a CamecaSX100
electron probe micro-analyzer (EPMA) (464 spot anal-yses from 225
mineral grains from 40 thin sections). Full resultsalong with
analyzing conditions, X-ray lines analysed and
cal-ibrationmaterials used are listed in Table S2. Standard
deviationbased on repeated standard measurements is 93
vol%accumulated crystals, mesocumulate 93–75 vol%, andorthocumulate
75–50 vol%. In the basal dunite, adcumulatesare typically dunites
and meso- and orthocumulates areharzburgites and lherzolites. A
heteradcumulate is a cumulatewith a poikilitic texture and a very
minor component of recog-nizable trapped intercumulus melt (Wager
et al. 1960; Barneset al. 2016b). The presence of trapped
intercumulus melt inolivine and orthopyroxene rich cumulates, like
found inNäränkävaara, can usually be recognized from elevated
contentsof incompatible elements (e.g., Al, Ti, P, Zr) in
whole-rockanalysis. As such, adcumulate and heteradcumulate rocks
tendto exhibit adcumulate chemistries poor in incompatible
ele-ments, and orthocumulate rocks tend to exhibit
orthocumulatechemistries relatively rich in incompatible
elements.
Petrography of the basal dunite
Ortho- and mesocumulates of the SW border zone
A 200–400 m thick border zone composed of olivine ortho-and
mesocumulates and minor adcumulates (units #1–2 in
41The basal dunite of the Precambrian mafic-ultramafic
Näränkävaara intrusion: Petrogenetic considerations...
-
Table1
Mainpetrographicalandgeochemicalcharacteristicsof
lithologicalu
nitsof
theNäränkävaarabasald
unite
Petrography
Geochem
icalzone
(Subzone)
Unit
#Height(m)Petrography
Geochem
istry
Mg#
1Fo
inolivine
(mol%)
Parentalmagma
MgO
3(w
t%)
Fo 9
0Fo 9
1
KD
KD
EPMA
Inferred
20.30
0.33
0.30
0.33
Contact
(0–20)
Strongly
alteredcontactzoneof
meta-peridotite,trem
olite-rock,
soapstone
Orthocumulateandmesocum
ulate
Low
-Fe(H
igh-Ni)
10–250
Oliv
ineorthocum
ulategradinginto
mesocum
ulatewith
distance
from
contact;intercum
ulus
consistsof
pyroxenes,minor
plagioclase,
biotite,trace
chromite,sulfides;
euhedral-subhedralchromite
Orthocumulatechem
istry
(incom
patib
leelem
entrich);
olivine-meltm
ixes;h
ighNi(aver-
age2500
ppm)
89.1
88.04
89.8
14.7
16.1
16.4
18.1
Orthocumulateandmesocum
ulate
Low
-Fe(M
oderate-Ni)
2Orthocumulatechem
istry;
olivine-melt
mixes;m
oderateNi(average
2000
ppm)
87.3
87.8
8913.0
14.5
14.8
16.5
Adcum
ulate
Low
-Fe(H
igh-Ni)
3250–550
‘Extreme’
olivineadcumulate,very
minor
intercum
ulus;1
–2vol%
fine-grained
euhedral-subhedral
chromite
Adcum
ulatechem
istry(incom
patible
elem
entp
oor);h
ighNi
Adcum
ulate
Low
-Fe(M
oderate-Ni)
4Adcum
ulatechem
istry;
moderateNi,
high
Cr(average
4000
ppm)
89.5
86.9
Orthopyroxene
adcumulate
Low
-Fe(?)
5550–625
Orthopyroxene
adcumulate(<5vol%
olivine)
Adcum
ulatechem
istry
88.5
Adcum
ulate
High-Fe
6625–725
‘Extreme’
olivineadcumulate,very
minor
intercum
ulus;lobateand
poikilitic
chromite
Adcum
ulatechem
istry;
lowNi
(average
1700
ppm)andCr
(average
1200
ppm)
87.2
86.3
Adcum
ulate
High-Fe
7725–750
Adcum
ulatechem
istry;
lowMgO
and
Mg#
forolivineadcumulate
85.7
85.1
Adcum
ulate
High-Fe
8750–1900?
Adcum
ulatechem
istry,lowNiand
Cr87
87.7
87.3
Oliv
ine-orthopyroxene
heteradcum
ulate
High-Fe
91900?–2500
Olivineheteradcum
ulatewith
sparse
cm-scaleorthopyroxeneoikocrysts
(about
5–20
vol%
dependingon
in-
tersection);p
oikiliticchromite
(low
erhalf)andeuhedral-subhedral
chromite
(upper
half)
Adcum
ulatechem
istry;
olivine-orthopyroxenemixes;
low-N
i;about2000ppmincrease
inCrassociated
with
change
from
poikilitic
toeuhedral-subhedral
chromite
86.6
87.6
87.3
Harzburgitead-andmesocum
ulateHigh-Fe
102275–2325
Interlayersof
texturally
variable
harzburgiticto
orthopyroxenitic
adcumulateandpoikilitic
mesocum
ulate
86.4
87.0
86.5
1Average
Mg#
forunitfrom
whole-rockXRFanalyses;Mg#
=Mg/(M
g+Fe
2+);FeO/Fe 2O3ratio
calculated
accordingto
Barnesetal.(2007),seetext;2Oliv
inecompositio
ndeterm
ined
from
linear
regression
ofwhole-rockMg/Alvs.Fe/Al(seeMakkonenetal.2017);3
Intersectio
non
aMgO
-FeO
diagram
ofawhole-rocksampleregression
lineandamodelliq
uidcompositio
ninequilib
rium
with
highestF
oolivinefound;
Param
etersFo 9
1–90andKD0.30–0.33wereused
tocalculatecompositio
nsof
differentequilibrium
liquids
(see
text
andFig.5);
4Fo
88.6from
boulder;EPMA=electron
probe
micro-analyzer,KD=Oliv
ine-meltM
g-Fe
exchange
coefficient
42 V. Järvinen et al.
-
Fig. 2 and Table 1) is found all along the SW contact betweenthe
basal dunite and the Archean basement complex. Thebasement contact
is subvertical or dips steeply to the NE(75–90 °). The
granite-gneiss basement is hornfelsed for adepth of at least 0.5 m,
and leached and broken for up to5 m from the contact. The immediate
contact zone is
-
#3–10 in Fig. 2 and Table 1). It is composed of
olivineadcumulates and minor orthopyroxenites in the SW (units
#3–8), followed by olivine-orthopyroxene heteradcumulate and mi-nor
orthopyroxenites in the NE (units #9–10). Olivineadcumulate
textures range from subhedral (Fig. 3c) to tightlypacked polygonal
(lower half of Fig. 3d). Olivine is typicallyequigranularwith grain
size between 0.5–3mm, but rare bimodalolivine is also found in the
NW block of the dunite (Fig. 3f). Theorthocumulate border zone
(units #1–2) most likely grades intothe adcumulates, but this
boundary is only intersected in one drillhole (R5), where it
coincides with a 1 m thick olivine-pegmatoidal layer, composed of
loosely packed cm-sizedsubhedral olivine pseudomorphs in a fine
grained serpentinegroundmass. No other magmatic layering has been
identified inthe adcumulates, and for the most part the inner
structure of theSE block of the basal dunite is poorly
constrained.
A < 200 m thick unit of fresh (olivine)
orthopyroxeneadcumulate (unit #5 in Fig. 2 and Table 1) is
foundsandwiched between two olivine adcumulate units.
Thisorthopyroxenite unit is notable in two ways. First, it marksthe
border between the geochemically defined low-Fe andhigh-Fe zones,
as discussed later. Second, a change in chro-mite textures seems to
be associated with the stratigraphiclevel of this unit. It has been
argued that all chromites inultramafic cumulates are cumulus in
origin, i.e., a liquidusphase, regardless of their crystal habit
(Barnes 1998; Godelet al. 2013), and thus the terms cumulus and
intercumulus areavoided for describing chromite textures. In this
study, chro-mites have been classified as euhedral-subhedral (upper
grainin Fig. 4b), interstitial (weakly growing between cumulus
grains; lower grain in Fig. 4b, also Fig. 3c), lobate
(moreextensive fingering between cumulus grains; Fig. 4c),
andpoikilitic (completely enclosing adjacent grains; Fig. 4d).
Aslabeled in Fig. 2c, below the orthopyroxenite unit (in units
#1–4) chromite textures are fine grained euhedral or
interstitial,whereas above the orthopyroxenite (in units #6–9)
chromitetextures are typically lobate or poikilitic. These
systematicscontinue up to the middle of unit #9, where chromite
texturechanges back to euhedral. In the basal dunite, olivine
cumu-lates with most abundant chromite (up to 1–2 vol%) alwaysshow
euhedral-subhedral chromite textures. Poikilitic chro-mite is
notably coarser (up to 2 mm) compared to euhedralchromite, but
appears in much lower total modal quantities.Units 7–9# contain the
coarsest lobate and poikilitic chromitesin the basal dunite (Fig.
4c and d).
The heteradcumulate unit (unit #9 in Fig. 2 and Table 1),found
along the basal dunite–layered series contact, has adistinct
poikilitic texture with sparse pseudomorphicorthopyroxene grains
2–5 cm in diameter enclosing olivinegrains (Fig. 3d), but otherwise
its petrography is similar tothe regular olivine adcumulates.
Midway up through the unit,chromite texture changes from coarse
poikilitic to more abun-dant fine-grained euhedral, and continues
like this until thelayered series contact (Fig. 2c). A relatively
heterogeneous50 m thick unit (unit #10) comprising 0.5–5 m thick
inter-layers of variable-textured harzburgitic to
orthopyroxeniticadcumulates and mesocumulates (Fig. 3e) is found
withinthe heteradcumulate unit, about 100 m before the layered
se-ries contact. The change from poikilitic to cumulusorthopyroxene
(and also chromite) observed from unit #9 to
Chro
mite
satu
ratio
n su
rface
s
Opx
Ol
FQMFQ
M -1
.5
Cpx
LobatePoikilitic
EuhedralChromite texture
Ol + Melt + (Chr)
Ol+Chr-cotecticadcumulates
Ol + (Chr)adcumulates
Ol+Opx+Chrperitecticcumulates
MS
700 um
500 um
2000 um
MgO (wt%)
Cr (
ppm
)
15 20 25 30 35 40 45
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Vetren
y
KellojärviModel PM
Euhedral
Lobate
Poikilitic
Marginal series (R1 173 m)
Layered seriesBase of Ultramafic zone (unit Peridotite-1)
Unit 1 - Olivine ortho- and mesocumulate, high-NiUnit 2 -
Olivine ortho- and mesocumulate, moderate-NiUnit 3 - Olivine
adcumulate, high-NiUnit 4 - Olivine adcumulate, moderate-Ni
Basal dunite - Low-Fe ZoneUnit 5 - Orthopyroxenite
Basal dunite - High-Fe zone
Unit 6 - Olivine adcumulateUnit 7 - Olivine adcumulate
(low-Mg#)Unit 8 - Olivine adcumulateUnit 9 - Olivine-orthopyroxene
heteradcumulateUnit 10 - Harburgite and bronzitite
a
c
b
d
Fig. 4 aWhole-rockMgOversus Cr compositions of Näränkävaara
basaldunite samples. Olivine adcumulate samples (units #3, 4, 6, 7,
8, and 9)represent either olivine cumulates or olivine chromite
cumulates, whereasthe SW border zone ortho- and mesocumulate
samples (units #1, 2) rep-resent olivine–melt mixes (±chromite)
(Barnes 1998). Labeled fieldsshow (1) modeled basal dunite parental
magma composition (“ModelPM”), and (2) comparisons to whole-rock
compositions from the
komatiitic basalts of the Vetreny Belt (Puchtel et al. 1997) and
thekomatiitic Kellojärvi ultramafic cumulate complex (Papunen et
al.2009). Colored fields indicate texture of chromites in thin
section (seelegend); b–c Back-scattered electron images of
chromites with thinchromian magnetite rims, number indicates length
of picture bottomedge; b lower grain euhedral, upper grain
interstitial (sample JSN-87), clobate (sample JSN-89), d poikilitic
(sample VJJA-81)
44 V. Järvinen et al.
-
unit #10 is in line with the common observation that
poikiliticphases precede the appearance of that phase as
cumulus(Campbell 1987).
A subvertical tectonized contact between the
olivineheteradcumulate (unit #9) and the Näränkävaara layeredseries
is intersected in drill hole R1 at a depth of~170 m. The contact
includes a 10 m thick non-cumulus textured melagabbronoritic
marginal series de-veloped on the side of the layered series
(bright red inFig. 2c) (Järvinen et al. 2020).
Whole-rock chemical compositions
Main results
Results of representative whole-rock analyses of theNäränkävaara
basal dunite are presented in Table 2. All sam-ples are MgO-rich
(24–47 wt%). Olivine adcumulates contain45–47.7 wt% MgO, with the
exception of one low-Mg#adcumulate unit with only 42 wt% MgO (unit
#6 in Table 1and Fig. 2). In the adcumulates, FeOt ranges between
9.0–13.2 wt%, and increases with decreasing MgO (Mg# 84.4–90.4;
Fig. 5). Mesocumulates contain 39–45.5 wt% MgO andorthocumulates
31–38 wt%MgO, but values down to 24 wt%are found in heavily altered
(olivine-pyroxenitic?) samplesclosest to the basement contact. The
heteradcumulates in drillholes R1 and R2 (unit #9) typically
contain 42–45 wt%MgO,grading down to 32–34wt% in the
orthopyroxenite interlayers(unti #10). On Pearce element ratio
plots of (Mg + Fe)/Al ver-sus Si/Al, all olivine cumulate and
heteradcumulate units ploton straight lines with slopes of 1.7–2.0.
This indicates strongolivine control of whole-rock compositions as
pure olivineplots on a slope of 2.0.
Based on olivine-compatible element concentrations inwhole-rock
samples, the basal dunite can be divided intotwo zones with
overlapping MgO contents (Fig. 6a): (1) amore primitive low-Fe zone
(units #1–5; downward-triangles in all diagrams), characterized by
relatively lowFeOt (average 10.2 wt%) and high Ni (average 2250
ppm),with MgO between 32 and 47.3 wt% (Mg# 85.3–90.4); and(2) a
more fractionated high-Fe zone (units #6–10; upward-triangles in
all diagrams) characterized by relatively high FeOt(average 12.5
wt%) and low Ni (average 1700 ppm), withMgO between 40 and 46.0
(Mg# 85.7–88.6). The low-Fe zonecan be further subdivided into a
high-Ni subzone with anaverage of 2500 ppm Ni (units #1 and #3;
pink in Fig. 6eand all other diagrams), and a moderate-Ni subzone
(units#2 and #4; red in Fig. 6e and all other diagrams) with
anaverage of 2000 ppm Ni. Average Al2O3/TiO2 is slightlyhigher in
the low-Fe zone (22) compared to the high-Fe zone(19), ranging
16–26.
As shown in Figs. 2 and 7, the low-Fe zone (units
#1–5)encompasses the ortho- and mesocumulate border zone, the
lowermost olivine adcumulates, and the lowermostorthopyroxenite
unit. The high-Fe zone (units #6–10) encom-passes the rest of the
basal dunite up to the layered seriescontact. The high-Ni subzone
of the low-Fe zone has onlybeen intersected in drill holes R6, R7,
and R9, and thus doesnot appear in the geochemical profile in Fig.
7.
The low-Fe and high-Fe zones both have variable Cr
concen-trations. Chromite texture roughly correlates with
whole-rock Crcontents (Fig. 4): poikilitic and lobate chromite is
found in high-MgO low-Cr cumulates, and euhedral-subhedral chromite
in high-Cr cumulates. Similar MgO-Cr systematics have been
describedfrom komatiites (Barnes and Hill 1995; Barnes 1998). In
the basaldunite, highestCr contents (2000–5160 ppm) are found in
sampleswith euhedral and/or weakly interstitial chromite (Fig. 4).
LowerCr contents (
-
Table 2 Representative results of whole-rock chemical analyses
from the Näränkävaara basal dunite, with a modeled parental magma
composition forthe basal dunite and comparison to the Näränkävaara
marginal series gabbronorite
Zone (subzone) Low-Fe (high-Ni) Low-Fe (moderate-Ni) High-Fe
Marginal Series1 Modeledparental magma2
Rocktype3 Per Serp Dun Dun Dun Dun Dun Hzb Mgbnor
Cumulus4 oMCba? obMCa*? oACba? oAC oAC oAC oMC(b*) obAC(a*)
abMCpf
Unit # 1 2 2 4 7 8 9 10
Sample5 1 2 3 4 5 6 7 8 9
Major oxides (wt%)
SiO2 45.15 46.44 42.23 41.60 43.50 41.44 41.49 46.43 54.28
50.70
TiO2 0.14 0.34 0.03 0.02 0.01 0.01 0.01 0.04 0.52 0.54
Al2O3 2.77 8.29 0.78 0.34 0.21 0.16 0.22 0.65 11.34 12.24
FeOt 9.85 9.65 11.49 9.99 13.46 12.29 12.46 11.34 9.42 9.23*
MnO 0.14 0.21 0.17 0.13 0.20 0.17 0.18 0.18 0.17 n.a.
MgO 41.61 28.83 45.16 47.69 42.58 45.49 45.13 40.49 13.79
15.59
CaO 0.14 5.99 0.10 0.20 0.05 0.44 0.45 0.86 7.20 8.82
Na2O 0.08 0.07
-
the trend lines coincide with measured olivine andorthopyroxene
compositions, and these units can be modeledas linear mixtures of
olivine and orthopyroxene. They are thusinterpreted as
heteradcumulates with no (or only minor)trapped intercumulus melt
(e.g. Barnes et al. 2016b).
Rare earth element compositions
Chondrite-normalized REE and primitive-mantle-normalized trace
element patterns for the basal dunitesamples are presented in Fig.
9. All adcumulates andheteradcumulates (units #3–10) are very
REE-poor, withLa between 0.5–1 times chondritic and most HREE
belowde t ec t i on l im i t s . The bo rde r zone o r tho -
andmesocumulates (units #1–2) are LREE-enriched with La-values
approximately 2–20 times chondritic. Negative Euand Sr anomalies
are likely related to Ca-mobility duringserpentinization. Total
REE-abundances increase with de-creasing MgO, which together with
the parallel REE-patterns suggests olivine control (dilution) of
REE. TheREE patterns are interpreted to mainly reflect the
amountand composition of the trapped intercumulus melt.
Chalcophile elements
Concentrations of chalcophile elements are generally low inthe
basal dunite, except for Ni, which must be almost entirelyhosted by
olivine judging from the overall low concentrationsof sulfur (Table
2). In the adcumulates and heteradcumulates(units #3–10), Cu, Pd,
Pt, and S are mostly below detectionlimits. The border zone
cumulates (units #1–2) contain onaverage 480 ppm S (highest 1300
ppm). Most Cu and abouthalf of Pd and Pt analysis results are below
detection limits;
highest measured Cu is 170 ppm, Pd is 5.7 ppb, and Pt is10.8
ppb.
Mineral chemistry
Chromite
Results of representative EPMA mineral analyses arelisted in
Table 3. All analyzed spinels from the basaldunite (39 samples, 119
grains, 453 analyses) are classi-fied as chromites after the most
abundant trivalent ion(Fig. 10), except for two samples that
contain three grainsclassified as spinels. Chromites have
compositions with(average in parentheses): Mg# 0.12–0.5 (0.32),
Cr#0.45–0.80 (0.63), Fe3+# 0.03–0.51 (0.12), NiO 0.03–0.22 wt% (0.1
wt%), and ZnO 0.08–1.43 wt%(0.4 wt%) [where Cr# = Cr/(Cr + Al) and
Fe3+# = Fe3+/(Cr + Al + Fe3+)]. There is large overlap in
compositions,but generally the ortho- and mesocumulates (units
#1–2)contain more chromian and ferrian chromites, while
theadcumulates (units #3–10) contain more aluminian chro-mites
(Fig. 10). Chromite core compositions are igneousbased on
constraints of Barnes and Roeder (2001).Chromite compositions
overlap with the compositionalfields of both layered intrusions and
komatiites (Fig. 10)(Barnes and Roeder 2001). No systematic
compositionalvariations between euhedral and lobate–poikilitic
chro-mite grains were found.
Chromites from ortho- and mesocumulates (units #1–2) exhibit a
wider range of Fe2O3 compositions (5–21.5 wt%) compared to those
from other units (average5.8 wt%) (Fig. 10a and c). In
back-scattered electron(BSE) imaging, the chromites do not exhibit
magmatic
Table 2 (continued)
Zone (subzone) Low-Fe (high-Ni) Low-Fe (moderate-Ni) High-Fe
Marginal Series1 Modeledparental magma2
Rocktype3 Per Serp Dun Dun Dun Dun Dun Hzb Mgbnor
V n.a. 89.9 n.a. n.a. n.a. 7.40 9.2 n.a. 150.0 151.1
Co n.a. 91.5 n.a. n.a. n.a. 140.00 138.0 n.a. 58.0 78.9
Au (ppb)
-
zoning, but thin chromian magnetite rims with Fe2O3replacing
Al2O3 are commonly found (Fig. 4b–d) espe-cially in the
orthocumulate samples. The orthocumulate
chromites also have higher contents of e.g. Mn, V, Zn,and Ni,
with abundances of these elements increasingwith decreasing
whole-rock MgO. Barnes (1998)interpreted similar compositional
trends in komatiitic cu-mulates as post-cumulus equilibration
between chromiteand trapped intercumulus melt during slow
cooling.Compositional variations in chromites along the
X-X’cross-section (Fig. 2) are illustrated in Fig. S2.
Olivine
Fresh olivine in the Näränkävaara basal dunite was foundin only
13 samples. Relatively abundant fresh olivine isfound in some
less-altered ortho- and mesocumulate sam-p l e s ( u n i t s # 1–2
) , a nd a l s o i n t h e ( o l i v i n e )orthopyroxenites (units
#5 and 10). In the adcumulatesand heteradcumulates olivine has only
been found intwo samples as smal l re l ic t cores in
otherwiseserpentinized pseudomorphs (units #8–9).
The analyzed olivine compositions range betweenFo87–88.6, with
two orthocumulate samples extendingdown to Fo84.5 (see Tables 1 and
3). Highest measuredfors ter i t e conten t of Fo88 . 6 i s f rom
an ol iv inemesocumulate boulder (unit #1); the second highest
ofFo88.5 is found in the lowermost orthopyroxenite (unit#5).
Analyzed olivines exhibit three different levels ofNi
concentrations (Fig. 11): (1) high-Ni from 0.40–0.37 wt% NiO; (2)
moderate-Ni from 0.30–0.28 wt%NiO; and (3) low-Ni from 0.28–0.21
wt% NiO. Theselevels mirror whole-rock Ni values and the
geochemicalsubdivision presented earlier (Fig. 6e). Compared to
astatistical analysis of komatiitic olivine compositions byBarnes
and Fiorentini (2012), the Ni-richest olivines fromthe low-Fe zone
(unit #1) can be classified as both” min-eralized komatiites” and
“barren units in mineralized do-mains”, while all other olivine
analysis results fall in thefield of “fresh olivine in
unmineralized komatiites”.
Pyroxenes
Orthopyroxene was analyzed from nine samples andc l inopyroxene
f rom seven samples (Tab le 3) .Orthopyroxene compositions range
between En83.7–87.0,Fs9.7–12.2, and Wo2.4–4.5; with Cr2O3 between
0.5–0.7 wt%and NiO between 0.05–0.11 wt%. Clinopyroxenes
rangebetween En46.7–52.1, Fs5.8–7.2, and Wo39.5–44.9; withCr2O3
between 0.65–1.30 wt% and NiO between 0.03–0.06 wt%. Pyroxenes from
the low-Fe zone contain uni-formly higher NiO compared to those of
the high-Fezone (e.g. average of 0.048 versus 0.035 wt% NiO
inclinopyroxene).
Model PM
89 90 91
Mod
el e
quilib
rium
liqu
ids
Fo91
Fo90
Fo89
Fo88
Fo87
Fo86
10 15 20 25 30 35 40 45 50MgO (wt%)
FeO
** (w
t%)
13
12
11
10
9
8
7
6
MS
Marginal series (R1 173 m)
Layered seriesBase of Ultramafic zone (unit Peridotite-1)
Unit 1 - Olivine ortho- and mesocumulate, high-NiUnit 2 -
Olivine ortho- and mesocumulate, moderate-NiUnit 3 - Olivine
adcumulate, high-NiUnit 4 - Olivine adcumulate, moderate-Ni
Basal dunite - Low-Fe Zone
Basal dunite - High-Fe zone
Unit 5 - Orthopyroxenite
Unit 6 - Olivine adcumulateUnit 7 - Olivine adcumulate
(low-Mg#)Unit 8 - Olivine adcumulateUnit 9 - Olivine-orthopyroxene
heteradcumulateUnit 10 - Harburgite and bronzitite
Fig. 5 Whole-rock MgO versus FeO** compositions of
Näränkävaarabasal dunite samples; FeO** is FeO calculated from
total FeO accordingto Barnes et al. (2007). Pure olivine
compositions Fo86–91 are plotted onthe right, and model liquids in
equilibrium with Fo89–91 olivines witholivine-liquid Fe-Mg exchange
coefficient (KD) values of 0.30 (solidblack lines) and 0.33 (dashed
black lines) (Roeder and Emslie 1970) areplotted on the left.
Figure illustrates modeling of the Näränkävaara basaldunite
parental magma composition: Units #1 and #2 are interpreted to
becomposed of olivine-melt mixtures; regression lines fitted
through theseunits (dashed pink line for unit#1 and red for unit
#2) intersect with Fo90equilibrium liquid compositions with KD
0.30–0.33 at 13–16.1 wt%MgO (black bar on x-axis). Field labeled
“Model PM” shows the rangeof modeled parental magma compositions
that results for units #1 and #2using a liquid composition in
equilibrium with Fo90 olivine at KD values0.30 and 0.33; note that
this field has been plotted in all other variationdiagrams in this
paper. Black dashed line connects compositions of oliv-ine and
liquid in equilibrium with the Näränkävaara marginal
seriesgabbronorite composition (Järvinen et al. 2020)
48 V. Järvinen et al.
-
Olivin
e, Lo
w-Fe s
ubzon
e
Olivine,High-Fesubzone
Opx MS
Ni (
ppm
)
FeO (wt%)t7 8 9 10 11 12 13 14
2750
2500
2250
2000
1750
1500
1250
1000
750
500
Low-FeHigh-Ni
High-FeLow-Ni
a
Marginal series (R1 173 m)
Layered seriesBase of Ultramafic zone (unit Peridotite-1)
Unit 1 - Olivine ortho- and mesocumulate,high-Ni
Unit 2 - Olivine ortho- and mesocumulate,moderate-Ni
Unit 3 - Olivine adcumulate, high-NiUnit 4 - Olivine adcumulate,
moderate-Ni
Basal dunite - Low-Fe ZoneUnit 5 - Orthopyroxenite
Basal dunite - High-Fe zone
Unit 6 - Olivine adcumulateUnit 7 - Olivine adcumulate
(low-Mg#)Unit 8 - Olivine adcumulate
Unit 9 - Olivine-orthopyroxeneheteradcumulate
Unit 10 - Harburgite and bronzitite
15 20 25 30 35 40 45
c
Opx
Vetre
ny
e
MS
Mod
el P
M
MgO (wt%)
2500
2000
1500
1000
500
Ni (
ppm
)
Opx
MgO (wt%)15 20 25 30 35 40 45
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
Vetreny
Kellojärvi
Cpx
Cpx
Opx
Ol
MS
MgO (wt%)
11
10
9
8
7
6
5
4
3
2
1
0
TiO
(wt%
)2
b
AlO
(wt%
)2
3
d
Vetreny
Kellojärvi
1213
15 20 25 30 35 40 45
0.55Model PM
Model PM
Ol
MS
Cpx
Norma
l olivin
e
Kellojä
rvi
Ol
Model
PM
CaO
(wt%
)
MgO (wt%)15 20 25 30 35 40 45
17.5
15.0
12.5
10.0
7.5
5.0
2.5
0.0
Model PM
Vetreny
Kellojärvi
Cpx
Opx
MS
Ol
Fig. 6 a–eWhole-rock compositions of Näränkävaara basal dunite
sam-ples on variation diagrams; a Dashed line shows the main
geochemicaldivision of basal dunite samples into low-Fe and high-Fe
zones. Fieldlabeled “Model PM” shows the range of modeled basal
dunite parentalmagma compositions. Note that the trend of the
border zone ortho- andmesocumulate samples (units #1–2) coincides
with the marginal series
gabbronorite of the layered series (labeled “MS”), suggesting
these rocksare mixtures of cumulus olivine and melt approximately
of the marginalseries composition. Other labeled fields show
measured mineral compo-sitions (Ol, Opx, Cpx) and comparisons to
whole-rock compositions fromthe komatiitic basalts of the Vetreny
Belt (Puchtel et al. 1997) and thekomatiitic Kellojärvi ultramafic
cumulate complex (Papunen et al. 2009)
49The basal dunite of the Precambrian mafic-ultramafic
Näränkävaara intrusion: Petrogenetic considerations...
-
Discussion
General remarks
Twomajor research questions concern the Näränkävaara
basaldunite. First is related to its age and relationship with
theneighboring layered series. In the SE-block of theNäränkävaara
intrusion, a marginal series gabbronorite sepa-rates the dunite
from the layered series (Fig. 2). Unless thecurrent structural
configuration is fully the result of post-emplacement faulting,
this means there has to be an age dif-ference between the two. The
basal dunite could either be (1)comagmatic with the Fennoscandian
2.5–2.4 Ga layered se-ries magmatism, with a long enough hiatus in
magmatism toallow for the cooling needed for the subsequent
formation ofthe marginal series, or (2) represent an Archean
komatiiticwall-rock to the Proterozoic layered series
magmatism.Since isotope dating methods have thus far proven
unsuccess-ful, here we aim to constrain the petrogenesis of the
basaldunite by studying its parental magma composition and
dif-ferentiation, and compare our findings to the neighboring
lay-ered series and to nearby komatiites. The second major
re-search question is related to the orthomagmatic Ni-potentialof
the basal dunite as, in any event, it must have formed from a
large volume of mantle derived primitive magma(s).
Someobservations related to the Ni-potential of the basal duniteare
discussed at the end of this section.
Parental magma
One way to study a possible comagmatic relationship betweenthe
Näränkävaara basal dunite and the layered series is to com-pare
their parental magma compositions. The Näränkävaara lay-ered series
parental magma has been inferred from a marginalseries gabbronorite
intersected in drill hole R1 (Fig. 2) (Järvinenet al. 2020). As the
composition of the basal dunite parentalmagma cannot be directly
inferred from a chilled margin orspinifex textured rock, instead
the parental magma compositionhas beenmodeled here using results of
whole-rock analyses fromorthocumulates. The methods used here have
been outlined indetail by e.g. Bickle (1982) and Chai and Naldrett
(1992), andare based on the assumption that olivine orthocumulate
rocks area mixture of cumulus olivine and melt in equilibrium with
thisolivine. First, the Mg# of a melt in equilibrium with the
mostprimitive olivine found in the basal dunite was calculated
byusing the olivine-melt Mg-Fe exchange coefficient (KD) ofRoeder
and Emslie (1970). Then, the MgO and FeO contentsof the parental
magma were calculated from the intersection
0 m100200300400500600700800900
1000110012001300140015001600170018001900200021002200230024002500
Cr (ppm)0 2500 5000
Ni (ppm)Chromite0 1000 2000 3000
Ni (ppm)Olivine0 1000 2000 3000
X
X’
Ni (ppm)0 1000 2000 30005025 30 35 40 45
MgO (wt%)80 82.5 85 87.5 90
Mg#
Whole-rock Mineral
Marginal series (R1 173 m)Layered series
Basal dunite - Low-Fe Zone
Unit 2 - Olivine ortho- and mesocumulate, moderate-NiUnit 4 -
Olivine adcumulate, moderate-NiUnit 5 - Orthopyroxenite
Basal dunite - High-Fe zone
Unit 6 - Olivine adcumulateUnit 7 - Olivine adcumulate
(low-Mg#)Unit 8 - Olivine adcumulateUnit 9 - Olivine-orthopyroxene
heteradcumulateUnit 10 - Harburgite and bronzitite
Unit
2
4 56
8
9
7
10
a b c d e f
Lobate andpoikilitic chromite
9
Fig. 7 a–dWhole-rock and e–f mineral chemical variations with
height (m) in the X-X’ cross section in the SE block of the
Näränkävaara intrusion inFig. 2, with 0 m marking the SW contact
between the basement complex and the basal dunite. Horizontal lines
show unit division according to Table 1
50 V. Järvinen et al.
-
Marginal series (R1 173 m)
Layered series
Unit 1 - Olivine ortho- and mesocumulate,
high-Ni
Unit 2 - Olivine ortho- and mesocumulate,
moderate-Ni
Unit 4 - Olivine adcumulate, moderate-Ni
Basal dunite - Low-Fe Zone
Basal dunite - High-Fe zone
Unit 5 - Orthopyroxenite
Unit 6 - Olivine adcumulate
Unit 8 - Olivine adcumulate
Unit 9 - Olivine-orthopyroxene
heteradcumulate
Gd
/ Y
b
Ce / Sm
MS
Kellojärvi
Närä
nkä
vaara
Vetreny
Model PM
Zr / Y
Al2O3 / TiO2
MS
Kellojärvi
När
änkä
vaar
a
Vetr
eny
Model PM
TiO2 (wt%)
Zr (p
pm
)
MS
Kel
lojä
rvi
Närä
nkävaara
Vetreny
Model PMY
/ A
l2O
3
Zr / TiO2
MS
Kel
lojä
rvi
Näränkävaara
Vetreny
Model PM
MS
Kello
järv
i
Närä
nkävaara
Vetreny
Model PM
Y (p
pm
)
Zr (ppm)
a
d
b
e
c
Fig. 8 a–e Whole-rock trace element compositions and ratios of
basaldunite samples. Field labeled “Model PM” shows the range of
modeledbasal dunite parental magma compositions; other labeled
fields showcomparisons to whole-rock compositions from the
Näränkävaara layeredseries (Järvinen et al. 2020), komatiitic
basalts of the Vetreny Belt
(Puchtel et al. 1997), and the komatiitic Kellojärvi ultramafic
cumulatecomplex (Papunen et al. 2009). Error bars show 1σ relative
errors forelement ratios, samples with 1σ errors larger than the
ratio have beenomitted
51The basal dunite of the Precambrian mafic-ultramafic
Näränkävaara intrusion: Petrogenetic considerations...
-
between this equilibrium-melt composition and a regression
linefitted through samples composed of olivine-melt mixtures,
asillustrated in Fig. 5. As the border zone ortho- andmesocumulates
(units #1–2) are interpreted to representolivine-melt mixtures,
they were used for calculating these re-gression lines.
Concentrations of other elements in the parentalmagma were then
estimated in a similar fashion, by fitting re-gression lines
through the ortho- and mesocumulate units andcalculating the value
of each element at the previously obtainedMgO value. Samples taken
from immediate contacts with base-ment or xenoliths were omitted
from modeling to reduce scatterinwhole-rock sample regression
lines, as these near-contact sam-ples show distinctly stronger
LREE-enrichment indicative of in-situ contamination (not
shown).
Themost primitive olivines (Fo88–88.6) are found in the
olivineortho- and mesocumulates of the low-Fe zone (units
#1–2).During post-cumulus processes, olivine compositions tend
toprogressively re-equilibrate towards lower Fo contents as
modalintercumulus increases (e.g. unit #2 in Fig. 11) (Barnes
1986). Itis thus unlikely that the olivines analyzed from the
relativelyintercumulus-rich ortho- and mesocumulates represent
originalcompositions. It is also possible that the most primitive
olivineshave simply been destroyed during serpentinization.
Therefore,average olivine compositions have also been inferred
fromwhole-rock compositions. For the adcumulate units, olivine
Focontents can be directly inferred from whole-rock Mg#. Thehighest
whole-rock Mg# of 90.4 is found in the lowermost oliv-ine
adcumulates of the low-Fe zone (unit #4 in Fig. 7a andTable 1). A
slightly higher Fo content can be inferred from theMgO versus FeO
plot in Fig. 5, where these samples plot close topure Fo91 olivine.
In addition, average Fo contents were inferredfrom molecular ratios
of whole-rock Mg/Al and Fe2+/Al (seeMakkonen et al. 2017), with
results listed in Table 1. Using thislatter method, results for the
high-Ni subzone (unit #1) are be-tween Fo89–90 and for the
moderate-Ni subzone (unit #2)
between Fo87–89, depending on the drill hole. Results of
thismethod indicate similar olivine Fo contents as regression
lineson variation diagrams intersecting pure model olivine
composi-tions (e.g. Figure 5), with the added benefit of the method
alsobeing applicable to intercumulus-poor olivine adcumulate
units.The most primitive olivine inferred from the basal dunite is
thusin the range Fo90–91.
Parental magma compositions have been modeled separate-ly for
the high-Ni and moderate-Ni ortho- and mesocumulates(units #1–2)
with a liquid composition in equilibrium withFo90–91, using a KD of
0.30 and 0.33 (Roeder and Emslie1970; Toplis 2005). Resulting
parental magma MgO variesbetween 13 and 18 wt% (average 15.6 wt%),
depending onthe unit used for regression and the parameters used to
calcu-late the equilibrium liquid (Fig. 5 and Table 1). The
high-Niortho- and mesocumulates (unit #1 in Table 1) always result
ina higher parental magma MgO estimate compared to themoderate-Ni
ortho- and mesocumulates (unit #2 in Table 1).This difference is
related to the slopes of trend lines on theMgO-FeO diagram in Fig.
5, with unit #1 having slightly in-creasing FeO with decreasing MgO
resulting in a higher MgOintercept. As the high-Ni subzone is
Ni-richer and also appearsto contain slightly more forsteritic
olivine (Table 1), it is plau-sible that it has formed from a
slightly more primitive magmacompared to the moderate-Ni subzone.
An “average” parentalmagma composition for these two units has been
modeledusing a Fo90 equilibrium liquid and KD of both 0.30 and
0.33(see Fig. 5) – the four resulting parental magma
compositionsare plotted as the corners of the “Model PM” field in
variationdiagrams in Figs. 4, 5, 6 and 8; with REE and trace
elementdiagrams in Figs. 8 and 9. An average model parental
magmacomposition is also listed in Table 2.
Overall, the modeled parental magma composition is thatof an
LREE-enriched high-MgO basalt, highly similar to themarginal series
gabbronorite of the Näränkävaara layered
La Pr Pm Eu Tb Ho Tm Lu
Ce Nd Sm Gd Dy Er Yb
100
10
1
0.1
Ch
on
drite
(N
aka
mu
ra
19
74
)
a
Heteradcumulate (Unit 9)
Adcumulate (Units 6, 8, 9)
Orthopyroxenite (Unit 5)
Basal dunite - Adcumulate zone
Unit 2, moderate-Ni
Unit 1, high-Ni
Basal dunite - Border zone ortho- and
mesocumulates
Marginal series (R1 173 m)
Modeled parental magma
Possible parental magmas
Vetreny belt
Kellojärvi komatiite complex
Comparisons
Ba U Ta Ce Sr Nd Sm Ti Y Lu
Rb Th Nb La Pr P Zr Eu Dy Yb
100
10
1
0.1
Prim
itiv
e m
an
tle
(M
acD
on
ou
gh
& S
un
19
95
) b
Fig. 9 a Chondrite-normalized whole-rock REE-diagram and
bprimitive-mantle-normalized trace element diagram for
Näränkävaarabasal dunite samples; normalization values from
Nakamura (1974) andMcDonough and Sun (1995), respectively. Also
plotted is the averagemodeled basal dunite parental magma
composition, for which verticalbars in a (for La, Sm, Yb) show
variation in model results based on
regression of either unit #1 (high-Ni) or unit #2 (moderate-Ni)
samples;note the similarity of modeled parental magma to the
Näränkävaara mar-ginal series gabbronorite. Grey and green filled
fields show comparisonsto whole-rock compositions from komatiitic
basalts of the Vetreny Belt(Kirichi and Vinela localities from
Puchtel et al. 1997), and the komatiiticKellojärvi ultramafic
cumulate complex (Papunen et al. 2009)
52 V. Järvinen et al.
-
Table 3 Representative results of EPMA chemical analyses and
calculated cation fractions
Chromites
Zone Low-Fe Low-Fe Low-Fe High-Fe High-Fe High-Fe High-Fe
Unit # 1 2 4 6 7 8 9
Rock type Peridotite Lherzolite Dunite Dunite Dunite Dunite
Dunite
Cumulus* oMCb? oOCb*a*(pf) oAC oAC oAC oCA oMC(b*)
Sample1 1 2 3 4 5 6 7
Major oxides (wt%)
SiO2 0.06 0.34 0.06 0.00 0.05 0.10 0.10
TiO2 0.52 0.46 0.51 0.42 0.58 0.51 0.43
V2O5 0.15 0.50 0.13 0.19 0.18 0.20 0.26
A12O3 16.34 10.82 17.68 14.24 19.63 22.26 22.68
Cr2O3 45.01 38.84 46.82 50.09 44.49 41.55 42.99
FeOt 30.76 42.48 26.34 28.36 27.49 26.57 23.80
MnO 0.41 0.34 0.23 0.18 0.34 0.35 0.12
MgO 5.26 4.04 7.50 5.36 6.53 7.51 9.23
NiO 0.09 0.16 0.12 0.03 0.07 0.08 0.08
ZnO 0.73 0.33 0.21 0.52 0.33 0.70 0.22
Total 99.33 98.30 99.59 99.42 99.69 99.84 99.92
Calculated cation fractions (apfu)2
Fe3+/∑trivalent 0 10 0 31 0 10 0 05 0 05 0 06 0 04Cr/∑ trivalent
0.59 0.49 0.58 0.66 0.57 0.52 0.54Al/∑ trivalent 0.32 0.20 0.32
0.28 0.38 0.42 0.42Mg# 0.26 0.19 0.34 0.27 0.32 0.36 0.43
Cr# 0.65 0.71 0.64 0.70 0.60 0.56 0.56
Olivines
Zone Low-Fe Low-Fe High-Fe High-Fe
Unit # 1 2 8 9
Rockname Peridotite Peridotite Serpentinite Dunite
Cumulus* oOCba? o(c)OCb*(a*) oAC oMC(b*c*)
Sample3 1 2 3 4
Major oxides (wt%)
SiO2 41.21 40.43 40.48 40.63
Al2O3 0.009 0.013 0.022 0.007
Cr2O3 0.015 0.019 0.025 0.021
FeOt 11.85 11.87 11.87 11.95
MnO 0.204 0.186 0.187 0.185
MgO 46.74 47.39 47.37 47.27
CaO 0.030 0.072 0.078 0.059
NiO 0.405 0.309 0.278 0.245
CoO 0.016 0.016 0.018 0.020
Total 100.48 100.31 100.32 100.39
Mg# 87.55 87.68 87.68 87.58
Pyroxenes
Zone Low-Fe Low-Fe High-Fe High-Fe
Unit # 2 2 5 8
Rockname Peridotite Peridotite Bronzitite Dunite
Cumulus* o(c)OCb*(a*) o(c)OCb*(a*) bAC oAC
Sample4 1 1 2 3
Mineral5 Opx Cpx Opx Cpx
53The basal dunite of the Precambrian mafic-ultramafic
Näränkävaara intrusion: Petrogenetic considerations...
-
a b
Basal dunite - High-Fe zone
Unit 5 - OrthopyroxeniteUnit 6 - Olivine adcumulateUnit 7 -
Olivine adcumulate (low-Mg#)Unit 8 - Olivine adcumulateUnit 9 -
Olivine-orthopyroxene heteradcumulateUnit 10 - Harburgite and
bronzitite
Unit 1 - Olivine ortho- and mesocumulate, high-NiUnit 2 -
Olivine ortho- and mesocumulate, moderate-NiUnit 3 - Olivine
adcumulate, high-NiUnit 4 - Olivine adcumulate, moderate-Ni
Basal dunite - Low-Fe Zone
Previous analyses (Alapieti 1982, Telenvuo 2017)Other
Great Dyke
ForrestaniaKomatiite
Kellojärvikomatiite
MtKeithKomatiite
Burakovsky
KoillismaaWestern Intrusions
Mg#
Cr#
Great DykeForrestaniaKomatiite
Kellojärvikomatiite
MtKeithKomatiite
Burakovsky
KoillismaaWestern Intrusions
Mg#
3+Fe
#
Great Dyke ForrestaniaKomatiite
Kellojärvikomatiite
MtKeithKomatiite
Burakovsky
KoillismaaWestern Intrusions
Al 3+Fe
Cr c
Fig. 10 a-c Chromite compositions in the Näränkävaara basal
dunite, withgrey circles showing previous results (Alapieti 1982;
Telenvuo 2017).Labeled fields show comparisons to chromites in
komatiites in green(Barnes et al. 1996) and to mafic-ultramafic
layered intrusions in red
(Wilson 1982; Chistyakov and Sharkov 2008; Karinen 2010; Bailly
et al.2011). Results of chromite analyses have been recalculated
assuming stoi-chiometry to an ideal XY2O4 formula according to
Barnes (1998); Mg# =Mg/(Mg + Fe2+); Cr# =Cr/(Cr +Mg); Fe3+# =
Fe3+/(Fe3++Mg +Cr)
Table 3 (continued)
Major oxides (wt%)SiO2 55.97 51.59 55.80 52.53TiO2 0.08 0.71
0.07 0.37Al2O3 1.52 3.03 1.08 3.91FeOt 7.59 4.27 7.29 3.51MnO 0.18
0.15 0.20 0.11MgO 33.05 18.30 33.74 16.85CaO 1.88 20.17 1.41
20.84Na2O 0.00 0.44 0.00 0.72Cr2O3 0.69 1.05 0.60 1.03NiO 0.09 0.04
0.07 0.03Total 101.15 100.00 100.36 100.00
Calculated cation fractions (apfu)6
Mg# 0.89 0.88 0.89 0.90En 85.31 51.07 86.67 48.43Fs 11.20 6.87
10.72 5.83Wo 3.50 40.46 2.60 43.05
FeOt = total FeO
* Cumulus nomenclature after Irvine (1982), with exceptions that
intercumulus phases marked as suffix, minerals
-
series (Figs. 8 and 9, and Table 2; marginal series plotted as
astar in all variation diagrams) (Järvinen et al. 2020). The
mar-ginal series gabbronorite has a Mg# of 74 and thus is in
equi-librium with Fo90–90.6 olivine (KD 0.30–0.33), similar to
themost primitive olivine inferred from the basal dunite (Fig.
5).These results suggest a comagmatic origin for the basal
duniteand the Näränkävaara layered series.
Large uncertainty to the model results presented here iscaused
by scatter in whole-rock MgO and FeO contents(Fig. 5). Some of this
may be related to iron mobility duringserpentinization, as
secondary magnetite veinlets are relativelycommon in drill core.
Several other significant sources of errorare also related to the
methods used here, discussed in moredetail in e.g. Arndt et al.
(2008).
Inner structure and differentiation of the basal dunite
Layered structure
The same succession of cumulates and geochemical variationsare
found in both the SE and NWblocks of the basal dunite, intwo
separate cross-sections 25 km apart (Fig. 2a) (this studyand
Järvinen et al. 2020). In cross-section in both blocks, fromSW to
NE, ortho- and mesocumulates are followed byadcumula tes and f ina
l ly o l iv ine or thopyroxeneheteradcumulates before the layered
series contact.Geochemically, both begin with a low-Fe zone
followed bya high-Fe zone, with whole-rock and olivine
compositionsshowing decrease in Mg# and Ni contents towards the
layeredseries (Figs. 7a and c; see also Fig. S1). The sequence of
high-and low-Cr cumulates (Fig. 7d), reflecting the amount
ofeuhedral (cumulus) chromite, is also the same on both
blocks.These observations indicate that the structure of the basal
du-nite is at least somewhat layered and remains similar over the30
km strike length of the intrusion.
Ortho- and mesocumulates of the border zone
Olivine orthocumulates are found in both komatiite flows
andmafic-ultramafic layered intrusions (Campbell 1987; Hill et
al.1995). Orthocumulate units of similar thickness as in the
basaldunite (Fig. 2c) are common along the margins of
komatiiteflows, with modal olivine increasing inwards with distance
fromthewall-rock contact (Gole andBarnes 2020). In comparison,
theAganozero-block of the ~2.44 Ga Burakovsky layered intrusionin
Russian Karelia (Fig. 1; Amelin et al. 1995) contains a 100–200 m
thick olivine orthocumulatic marginal sequenceinterpreted as a
marginal series for the intrusion (Nikolaev andAriskin 2005), with
similar modal and geochemical variation asfound in the Näränkävaara
ortho- and mesocumulates. As thelow-Fe zone represents the most
primitive part of theNäränkävaara basal dunite – and as the
orthocumulates of thelow-Fe zone can bemodeled as mixtures between
themost prim-itive olivine found and a trapped melt in equilibrium
with thisolivine – we suggest that the ortho- and mesocumulate
borderzone (units# 1–2) represents a marginal series of sorts
similar tothe referred examples. Orthocumulate textures have likely
devel-oped by relatively rapid cooling of olivine-bearing
magmaagainst a cool wall-rock leading to early crystallization
ofintercumulus minerals (Campbell 1987).
Adcumulates and heteradcumulates
Over 90% of the ~1.5–2.5 km thick Näränkävaara basal
duniteconsists of olivine adcumulates (Fig. 2c). Olivine
adcumulatesare thought to be formed by in-situ crystallization on
thecrystal-liquid interface, with continuous low-porosity
crystalgrowth facilitated either by diffusion combined with
High-Fe zone
Low-Fe zone
Unit 1 - Olivine ortho- and mesocumulate, high-Ni
Unit 2 - Olivine ortho- and mesocumulate, moderate-Ni
Unit 5 - Orthopyroxenite
Unit 8 - Olivine adcumulate
Unit 9 - Olivine-orthopyroxene heteradcumulate
Unit 10 - Harzburgite and orthopyroxenite
NiO
(w
t%
)O
livin
e
Fo (mol%)
80 81 82 83 84 85 86 87 89 9088
0.45
0.40
0.35
0.30
0.25
0.20
0.15
Boulder
1
2
3
Fig. 11 Olivine Fo versus NiO compositions in the Näränkävaara
basaldunite. The three compositional groups in olivines (high-,
moderate-, andlow-Ni) reflect whole-rock MgO versus Ni compositions
(see Fig. 6e),with Ni-richest olivines found in the border zone
ortho- andmesocumulates (units #1–2). Post-cumulus re-equilibration
of olivinewith trapped liquid typically results in trends of
decreasing Fo as shownby unit #2 (Barnes 1986). Olivine fractional
crystallization models shownin dashed black lines (modeled with
COMAGMAT 5.5.2; Fe2O3 of 0.1 ×FeO; oxygen fugacity buffered at
quartz-magnetite-fayalite; Ariskin andBarmina 2004) for average
modeled parental magma compositions of (1)unit #1 of the basal
dunite, (2) unit #2 of the basal dunite; and for the (3)marginal
series gabbronorite of the Näränkävaara layered series (Järvinenet
al. 2020)
55The basal dunite of the Precambrian mafic-ultramafic
Näränkävaara intrusion: Petrogenetic considerations...
-
convective extraction of solute in near-equilibrium low-heatflux
conditions (e.g., slow-cooling magma chamber)(Campbell 1987; Walker
et al. 1988) or by turbulent flow oflow-viscosity magma flushing
away the depleted solute fromthe crystal-melt interface (e.g.,
komatiite flow) (Hill et al.1995).
The adcumulate units of the Näränkävaara basal dunite exhib-it
some features typically described from high-volume
turbulentkomatiite flows. In the low-Fe zone, adcumulates contain
smoothsinusoidal fluctuations inMg#with height (Fig. 7a, see unit
#4 at350–550 m). Similar fluctuations in komatiitic cumulates
havebeen interpreted as evidence of crystallization in an open
systemwith continuous influx of magmas with variable MgO
content(Arndt et al. 2008). In addition, the high-Fe zone
adcumulatescommonly contain poikilitic chromite (Fig. 4c-d), and
rarely alsobimodal olivine (Fig. 3f). Both are thought to form in
opensystem turbulent flows in komatiite settings (Barnes 1998;
Goleand Barnes 2020). Poikilitic chromite in adcumulates cannotform
from trapped intercumulus liquid alone, as the solubilityof Cr in
basaltic and komatiitic magmas is low (~3000 ppm).Rather,
poikilitic chromite is thought to crystallize competitivelywith
olivine in conditions of chromite supersaturation butinhibited
nucleation (Godel et al. 2013). Such conditions mayexist in freely
convecting magma chambers or channels underconditions of low
supercooling, where chromite supersaturationis reached only locally
and transiently; e.g. in hot turbulentkomatiite flows (Barnes
1998). In contrast, euhedral chromite istypically found in more
static settings with low cooling rates, e.g.in differentiated flows
(Barnes 1998) and in layered intrusions. InArchean komatiites,
poikilitic chromite is typically found inadcumulates with Fo92–93
and euhedral chromite below Fo91,with the high Fo contents
associated with poikilitic chromiteindicating high crystallization
temperatures close to chromiteliquidus (Barnes 1998). Here, those
Fo limits seem to have beenshifted down about 3 mol% (Table 1). As
far as we know, theseare the lowest Fo rocks where poikilitic
chromite has been re-ported. An additional complication is that,
unlike in komatiites,poikilitic chromite in Näränkävaara appears to
be associated withthe lowest Fo olivine adcumulates rather than the
highest(Table 1), implying that some other factor than temperature
hasinhibited chromite nucleation. Also, althoughmodeling indicatesa
relatively high liquidus temperature between 1300 and 1415 °Cfor
the basal dunite parental magma (COMAGMAT 5.5.2 withoxygen fugacity
buffered at quartz-fayalite-magnetite; Ariskinand Barmina 2004),
that is close to or above chromite liquidus(~1350 °C; Murck and
Campbell 1986), it is not clear whethersuch a dynamic turbulently
flowingmagma system as prescribedby Barnes (1998) could exist in an
intrusive environment. In anintrusive setting, poikilitic chromite
has only been described fromthe Dumont sill in Canada (Duke 1986).
Resolving how thepoikilitic chromite in Näränkävaara formed is
outside the scopeof this paper, but its ubiquitous presence in the
high-Fe zoneadcumulates in association with bimodal olivine
suggests the
possibility that these adcumulates were at least partly formed
ina high flow-through environment.
Whether the adcumulates of the basal dunite were formed in
ahigh-volume system or not, the composition of the in-situ
formedolivine should reflect the composition and differentiation of
themelt from which it formed. Generally, whole-rock Mg# and Ni(as
well as olivine and chromite Ni) decrease from SW to NEtowards the
layered series contact (Fig. 7), suggesting differenti-ation by
olivine fractionation from Fo90–91 down to Fo87–86.There are,
however, at least two distinct breaks or reversals indi-cated by
Mg#. The first break is marked by the lowermostorthopyroxenite unit
that separates the low-Fe and high-Fe zones(unit #5 in Fig. 7a),
and the second by a relatively low-Mg#adcumulate in the high-Fe
zone (unit #7 in Fig. 7a). At leastthe latter is likely related to
an influx of more primitive magma.
Difference in average compositions between the low-Fe andhigh-Fe
zones is quite large in regard toMgO, FeOt, and Ni (Fig.6a and e).
The olivine Fo-content at the beginning oforthopyroxene
crystallization also differs between the two zones.Orthopyroxene
first appears on the liquidus in the low-Fe zone atFo89
(orthopyroxenite unit #5 in Fig. 7a), and a second time inthe
high-Fe zone at Fo87.5 (heteradcumulate unit #9). This indi-cates
slightly different liquid lines of descent, and suggests amore
significant change in parental magma composition betweenthe low-Fe
and high-Fe zones (fractionation in staging cham-ber?). The low-Fe
zonemay represent a single magmatic system,beginning with formation
of the border zone orthocumulates(units #1–2), followed by possible
open system flow andformation of adcumulates (units #3–4), and
ending inclosed system pooling, fractionation, and formation ofthe
lowermost orthopyroxenite (unit #5).
Similar olivine orthopyroxene heteradcumulates as in unit
#9(Fig. 2c) are found in a similar position (preceding the
layeredseries contact) in the NW block of the intrusion, 25 km to
theNW (Järvinen et al. 2020). In the NW block, theheteradcumulates
are in contact with, or grade into, the basalharzburgite of the
Näränkävaara layered series. These NW con-tact zone
heteradcumulates and layered series basal harzburgitecompositions
are plotted as grey crosses in variation diagrams inFigs. 4, 5 and
6. These compositions closely match those of units#9 and #10 in the
SE block. At least for the high-Fe zone, thissuggests the basal
dunite and layered series magmas were fol-lowing identical liquid
lines of descent, having been locked-in tothe same
olivine-orthopyroxene-(chromite?) peritectic at an oliv-ine
composition of approximately Fo87.5.
Archean or Proterozoic origin?
The parental magma composition and liquid line of descent ofthe
Näränkävaara basal dunite seem to be strikingly similar tothat of
the neighboring 2.44 Ga mafic-ultramafic layered se-ries. However,
the basal dunite exhibits many features typicalof Archean
komatiitic cumulates, except with distinctly lower-
56 V. Järvinen et al.
-
Fo olivine as noted above. As the Näränkävaara intrusion
islocated between the Takanen and Suomussalmi greenstonebelts (Fig.
2a), both containing Archean komatiites (Iljina2003; Lehtonen et
al. 2017), the hypothesis that the dunitebody could represent an
older komatiitic wall-rock for thelayered series must also be
considered.
The nearby Suomussalmi and Takanen komatiites are rela-tively
thin olivine±clinopyroxene cumulates and flows with av-erage
Al2O3/TiO2 ratios of 17–19. Chondrite-normalized REE-patters are
flat or LREE-depleted (Papunen et al. 2009; V.Järvinen unpublished
data), except for the Vaara locality whichshows roughly similar
LREE-enriched patterns as theNäränkävaara basal dunite (Konnunaho
et al. 2013). Vaara isthe only nearby komatiite body with similar
high-Mg and low-Cr olivine adcumulates as found in Näränkävaara
(see Fig. 4),but it is significantly smaller in volume. The
uniformity of theinner structure of the Näränäkävaara basal dunite
along a strikelength of 30 km would only be expected in proximal
conduit orlarge scale channel facies komatiite flows (Gole and
Barnes2020). The 2.79 Ga Kellojärvi ultramafic cumulate
complex,located about 100 km south of Näränkävaara in the
Kuhmogreenstone belt of the Suomussalmi-Kuhmo-Tipasjärvi
green-stone complex, is the closest comparison of komatiitic origin
tothe basal dunite (Papunen et al. 2009; Lehtonen et al. 2016).
TheKellojärvi complex is 3–5 km thick and primarily composed
ofmetamorphosed olivine ortho-, meso-, and adcumulates withsimilar
petrography as in Näränkävaara, with the exception ofclinopyroxene
being the primary pyroxene instead oforthopyroxene. The general
lithology of the Kellojärvi complexis also similar, with a < 50
m thick border zone composed ofolivine orthocumulate and minor
pyroxenites (with basementxenoliths), grading into dunite towards
the center of the body(Tulenheimo 1999; Papunen et al. 2009).
Highest measured ig-neous olivine Fo contents (Fo82–89) (Tulenheimo
1999) andwhole-rock geochemistry are quite similar to
Näränkävaara(Fig. 6). Kellojärvi parental magma composition has
been in-ferred from whole-rock compositions to have 23 wt%
MgO(Makkonen et al. 2017), and the highest Fo content inferred
fromwhole-rock Mg#‘s is about Fo92, which are both somewhathigher
compared to the estimates from Näränkävaara presentedhere. These
differences, aswell as the differences in trace elementratios
(Figs. 8 and 9), could be explained by Näränkävaara hav-ing formed
from a more strongly differentiated and crustallycontaminated
komatiitic magma.
One observation against a komatiitic origin is that com-pared to
typical komatiitic olivine (Arndt et al. 2008) the ol-ivines from
the Näränkävaara basal dunite have distinctly low-er Cr2O3 (
-
with a uniform origin. Näränkävaara may have acted as amagmatic
feeder channel related to the Koillismaa layeredintrusion complex
and the “Hidden dyke” (Fig. 1) as sug-gested by Alapieti (1982),
and more generally as a magmaticcenter related to the Fennoscandian
LIPs as suggested byKulikov et al. (2010). In terms of their
composition, volcanicrocks found in the Saari-Kiekki greenstone
belt in easternFinland (Luukkonen 1989) could be viewed as
differentiatesof the Näränkävaara basal dunite and layered series
parentalmagmas (Järvinen et al. 2020; this study). Although
thesevolcanic rocks have not been precisely dated, they may
rep-resent extrusive equivalents of the Finnish 2.44 Ga
layeredintrusions. Nevertheless, extrusive rocks of this age
grouphave not been definitively identified from Finland, so it
ispossible that either (1) the magmatic systems never vented,(2)
they have been eroded away, or (3) the lavas all flowedinto the
Karelia-Kola rift now located in Russian Karelia.
Although we prefer the origin of the Näränkävaara basaldunite in
relation to the layered series in the light of the pre-sented
evidence and discussion, we emphasize that its petro-genesis is not
settled in the absence of absolute age or otherisotopic data, which
have proven difficult to acquire from theserpentinized ultramafic
rocks.
Mineral potential for orthomagmatic Ni-(Cu-Co-PGE)deposits
The Näränkävaara basal dunite represents a large volume
ofultramafic cumulates derived from a mantle plume source in anarea
with large trans-crustal structures, and its mineral potentialfor
Ni-(Cu-Co-PGE) is of great interest (Barnes et al. 2016a).Similar
Cr-poor olivine cumulates as found in Näränkävaara(Fig. 4) have
been found prospective for komatiite-hosted Ni-Cu-PGE deposits in
Finland (Konnunaho 2016).
There is a distinct difference in average whole-rock Nicontents
between the low-Fe zone (2200 ppm Ni) and thehigh-Fe zone (1780 ppm
Ni) (Fig. 6e). This Ni-depletion isreflected in olivine
compositions (Fig. 11; 3200 ppm versus2150 ppm Ni), chromite (1500
ppm versus 390 ppm Ni), andto a lesser extent also in pyroxenes.
Because the Ni-depletionappears in samples with the sameMg# and
forsterite contents,it cannot be related to simple fractionation of
olivine, even ifthe trapped liquid shift to lower Fo contents is
taken intoaccount (Fig. 10) (Barnes 1986). Rather, this suggests
eithera more Ni-rich parental magma for the low-Fe zone comparedto
the high-Fe zone, or that the Ni-depletion is caused bysulfide
saturation and segregation at some stratigraphic level.
No significant amounts of magmatic sulfides have beenfound in
the Näränkävaara area. Trace sulfides are commonlyfound in the NW
border zone olivine ortho- andmesocumulates. A basaltic magma
becomes sulfur-saturatedat approximately 1000 ppm S (Li and Ripley
2005).Assuming total incompatibility of sulfur in olivine, an
olivine
cumulate with 10 vol% trapped intercumulus melt (and90 vol%
olivine) would, at sulfur saturation, contain approx-imately 0.10 ×
1000 = 100 ppm S. Of the border zone samples(units #1–2), 62 out of
69 are above the detection limit of100 ppm S, containing between
100 and 1200 ppm S (averageof 480 ppm). These samples contain about
75–95 vol% cu-mulus olivine, suggesting that the trapped
intercumulus meltcontained in the samples may have been sulfur
saturated at thetime of emplacement. However, Cu, Pd, and Pt do not
corre-late with increasing S contents, suggesting that these
sulfidesequilibrated with a small volume of magma and
possiblyformed after emplacement.
Pt, and to a lesser extent Pd, behave similarly to
otherincompatible elements with their concentrations increasingwith
decreasing cumulus olivine (Fig. 12). Only 8 samples,all from the
moderate-Ni orthocumulate (unit #2), are abovedetection limit for
Cu and Pd, with Cu/Pd ratios between 8000and 40,000 (Fig. 12d).
These are above primitive mantle ratioor about 8000 (McDonough and
Sun 1995) which suggestssulfur saturation before emplacement.
Ratios of Pd/TiO2 andPt/TiO2 in the basal dunite are generally
below primitive man-tle ratio (Fig. 12c), also suggesting either
previous sufide sat-uration or sulfide retention in the source. Few
samples fromthe high-Ni border zone orthocumulates (unit #1) show
ratiosabove primitive mantle, which together with the generallymore
primitive nature of this unit suggests that it may haveformed from
a less PGE-depleted magma compared to unit #2.However, as most
assays are below or close to detection limitswith high relative
errors, more precise analyses are required toassess the sulfide
saturation history of the basal dunite in moredetail.
Conclusions
The Näränkävaara basal dunite is a 1.5–2 km thick series
ofultramafic cumulates stratigraphically below the 2.44
GaNäränkävaara layered series. It contains a 200–300 m thick
bor-der zone along the southern basement contact composed of
oliv-ine meso- and orthocumulates, but is otherwise composed
ofolivine adcumulates with lesser poikilitic
olivine-orthopyroxeneheteradcumulates andminor orthopyroxenites.
Textural and geo-chemical layering in the basal dunite remains
similar for the30 km strike length of the body. Geochemically, the
dunite iscomposed of a more primitive low-Fe zone along the
basementcontact (average FeOt of 10.2 wt% and Ni of 2250 ppm
withMg# 85.3–90.4), and a more evolved high-Fe zone in contactwith
the layered series (average FeOt of 12.5 wt% and Ni of1700 ppm with
Mg# 85.2–87.7). A distinct Ni-depletion in oliv-ine is found in the
low-Fe zone to the high-Fe zone (3200 versus2200 ppm Ni). This
depletion does not correlate with olivine Focontents, suggesting it
is not related to olivine fractionation.
58 V. Järvinen et al.
-
Hence the basal dunite may have potential for
Ni-(Cu-Co-PGE)sulfide mineralization.
The basal dunite parental magma composition has beenmodeled
based on olivine–melt mixing lines inferred fromthe border zone
ortho- and mesocumulates. It is an LREE-enriched high-MgO (13–18
wt.%) basalt that exhibits highlysimilar major and trace element
composition with the parentalmagma of the neighboring layered
series.
The presence of abundant low-porosity adcumulates,sometimes with
poikilitic chromite and bimodal olivine, to-gether with pyroxenitic
interlayers and at least two composi-tional reversals, suggests
formation in a dynamic magmaticsetting with high flow-through of
picritic magma, interruptedby periods of pooling and
differentiation. Differences in com-positions of the low-Fe and
high-Fe zones imply a change inparental magma composition with
time.
The presence of a marginal series between the basal duniteand
the layered series indicates that the basal dunite is older
than the layered series. The basal dunite is suggested to
haveformed in an early magmatic feeder channel related to thesame
Fennoscandian plume-related magmatism that producedthe 2436 Ma
Näränkävaara layered series and the other lay-ered intrusions of
the terrane. However, an alternative originas an Archean komatiitic
wall-rock cannot be ruled out.
Acknowledgements The Ni-Cu-Co-PGE projects of the
GeologicalSurvey of Finland are thanked for providing the materials
and analysesrequired for this study. All new whole-rock analyses
have been made atEurofins LabtiumOy laboratories, Kuopio, Finland,
and EPMAmeasure-ments were made at the Finnish Geosciences Research
Laboratory,Espoo, Finland. Lassi Pakkanen is thanked for his
diligent work on themineral analyses used in this study. Three
anonymous reviewers andeditor Lutz Nasdala are thanked for their
very helpful comments thatgreatly improved the manuscript. V.J. is
indebted to the K.H. Renlundfoundation for providing a PhD grant.
J.S.H. was supported by Academyof Finland Grant 295129.
Funding Open access funding provided byUniversity of Helsinki
includ-ing Helsinki University Central Hospital.
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Layered seriesMarginal series (R1 173 m)
Basal dunite - High-Fe zone
Basal dunite - Low-Fe ZoneUnit 5 - OrthopyroxeniteUnit 2 -
Olivine ortho- and mesocumulate, moderate-NiUnit 1 - Olivine ortho-
and mesocumulate, high-Ni
Unit 9 - Olivine-orthopyroxene heteradcumulateUnit 6 - Olivine
adcumulate
Pt (p
pb)
TiO (wt%)2
Cu
/Pd
Pd (ppb)
Pt/T
iO2
MgO (wt%)
PM
PM
1 2 3 4 5 10
15 20 25 30 35 40 45 0.0 0.1 0.2 0.3 0.4 0.5
1110
987654321
510
410
310
a b
Pt (ppb)1 2 3 4 5 10
Pt/T
iO2
210
10
d
210
10
PM
c
Fig. 12 a-d Whole-rock chalcophile element diagrams of