Download Petrogenesis of felsic igneous rocks associated with the

Petrogenesis of felsic igneous rocks
associated with the Paleoproterozoic
Koillismaa layered igneous complex, Finland
by
Laura S. Lauri
Division of Geology and Mineralogy
Department of Geology
University of Helsinki
P.O. Box 64
FI-00014 Helsinki, Finland
ACADEMIC DISSERTATION
To be presented, with the permission of the Faculty of Science of the
University of Helsinki, for public criticism in the small auditorium E204 of
Physicum, Kumpula, on May 28th, 2004, at 12 o’clock noon.
Helsinki 2004
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Supervisors: Prof. O.Tapani Rämö
Department of Geology
University of Helsinki, Finland
Prof. Emeritus Ilmari Haapala
Department of Geology
University of Helsinki, Finland
Reviewers:
Dr. Matti Vaasjoki
Geological Survey of Finland
Espoo, Finland
Prof. Eero Hanski
Department of Geology
University of Oulu, Finland
Opponent:
Dr. Mikko Nironen
Geological Survey of Finland
Espoo, Finland
cover: granophyre, sample LSL-00-83, crossed nicols.
ISBN 952-91-7207-9 (paperback)
ISBN 952-10-1853-4 (PDF)
Gummerus Kirjapaino Oy,
Saarijärvi 2004
2
Dedicated to Hannu and Silja
and the little one in prep.
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LAURI, L. S. 2004. Petrogenesis of felsic igneous rocks associated with the Paleoproterozoic
Koillismaa layered igneous complex, Finland. Academic dissertation, University of Helsinki,
Finland.
The Archean-Proterozoic transition at ~2.5–2.4 Ga was marked by worldwide anorogenic
magmatism that resulted in the emplacement of layered mafic intrusions, dike swarms, and
silicic plutonic rocks, as well as extrusion of volcanic rocks. These early Paleoproterozoic
rift-related rocks are found throughout the northern Fennoscandian Archean craton. In the
Koillismaa area, these rocks comprise the mafic Koillismaa layered igneous complex, the
Kynsijärvi quartz alkali feldspar syenite, and the volcanic Sirniö Group. In the adjoining
Oulanka area in northwestern Russia, a similar suite consisting of the Oulanka layered igneous
complex, the Nuorunen granite, and the Sumian volcanic rocks is found. U–Pb age
determinations indicate that the main magmatic evolution of the Archean complex in Koillismaa
took place at 2800–2700 Ma, whereafter the area was cratonized before the early
Paleoproterozoic rifting began. The Koillismaa layered igneous complex and the Kynsijärvi
quartz alkali feldspar syenite were emplaced into the Archean crust at ~2.44 Ga, and the
Sirniö Group is thought to be of approximately the same age. The Kynsijärvi pluton and the
felsic volcanic rocks of the Sirniö Group show A-type mineralogical and geochemical
characteristics such as high Fe/Mg in mafic silicates, fluorite as a conspicuous minor
component, high total alkali content, high Fe/Mg, Ga/Al, Zr, REE, and a negative Eu anomaly.
The Sirniö Group suffered from metamorphism and hydrothermal alteration that is thought to
coincide with the emplacement of the Koillismaa layered igneous complex beneath the
Sirniövaara Formation of the Sirniö Group. The Nd isotope composition of the Kynsijärvi
quartz alkali feldspar syenite (εNdi of –4.8 and TDM of ~3000 Ma) falls close to the evolution
path of the local Archean crust [εNd (at 2440 Ma) between –5 and –8.5 and TDM between 2900
and 3100 Ma], whereas the rocks of the Sirniö Group show values (εNdi of –1.9 and TDM
between 2800 and 2900 Ma) that are close to those acquired from the mafic layered intrusions.
The Nuorunen granite shows values (εNdi of –1.8 and TDM of 2750 Ma) that may reflect the Nd
isotope composition of local Archean crust in the Oulanka area. Geochemical modeling
utilizing immobile elements indicates that the early Paleoproterozoic silicic rocks could have
been derived from a mafic precursor by an AFC process involving a high-Mg parental magma
and lower crustal contaminants.
Keywords: Archean, Proterozoic, Koillismaa, Finland, A-type, layered mafic intrusion,
continental rifting, granophyre, volcanic rocks, U–Pb geochronology, Nd isotopes
Laura S. Lauri, Department of Geology, P.O. Box 64, FI-00014 University of Helsinki,
Finland
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Table of contents
Original publications............................................................................................................6
1. Introduction......................................................................................................................6
2. Previous studies................................................................................................................7
3. Regional geology..............................................................................................................7
3.1. Kynsijärvi quartz alkali feldspar syenite............................................................8
3.2. Lithostratigraphy................................................................................................9
3.3. Sampling and analytical methods.....................................................................10
4. Petrography.....................................................................................................................11
4.1. Kynsijärvi quartz alkali feldspar syenite..........................................................11
4.2. Sirniö Group.....................................................................................................11
4.2.1. Sirniövaara Formation........................................................................11
4.2.2. Unijoki Formation..............................................................................13
4.3. Nuorunen granite..............................................................................................13
4.4. Archean rocks...................................................................................................13
5. U–Pb geochronology......................................................................................................14
5.1. Kynsijärvi quartz alkali feldspar syenite..........................................................14
5.2. Archean rocks...................................................................................................14
6. Geochemistry..................................................................................................................15
6.1. Whole-rock geochemistry.................................................................................15
6.1.1. Kynsijärvi quartz alkali feldspar syenite............................................15
6.1.2. Sirniö Group.......................................................................................15
6.1.3. Nuorunen granite................................................................................17
6.1.4. Archean rocks.....................................................................................18
6.2. Nd isotope geochemistry...................................................................................18
6.2.1. Kynsijärvi quartz alkali feldspar syenite............................................18
6.2.2. Sirniö Group.......................................................................................18
6.2.3. Nuorunen granite................................................................................19
6.2.4. Archean rocks.....................................................................................19
6.3. Post-crystallization disturbance........................................................................19
6.4. Geochemical modeling.....................................................................................20
6.4.1. Boundary conditions..........................................................................20
6.4.2. Partial melting....................................................................................21
6.4.3. Fractional crystallization....................................................................22
6.4.4. Combined assimilation-fractional crystallization...............................25
7. Discussion.......................................................................................................................26
7.1. Geochronology of the Koillismaa area.............................................................26
7.2. The granophyre problem...................................................................................28
7.2. Early Paleoproterozoic volcanism in the northern Fennoscandian shield.........28
7.3. Early Paleoproterozoic silicic plutonism in the northern Fennoscandian
shield............................................................................................................29
8. Conclusions.....................................................................................................................30
Acknowledgments...............................................................................................................30
References...........................................................................................................................31
Appendix 1..........................................................................................................................36
Papers I - III
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Original publications
The thesis is based on following articles, referred to in the text by their Roman numerals.
I:
Lauri, L.S., Mänttäri, I., 2002. The Kynsijärvi quartz alkali feldspar syenite,
Koillismaa, eastern Finland – silicic magmatism associated with 2.44 Ga continental
rifting. Precambrian Res. 119, 121–140.
II:
Lauri, L.S., Karinen, T., Räsänen, J., 2003. The earliest Paleoproterozoic supracrustal
rocks in Koillismaa, northern Finland – their petrographic and geochemical
characteristics and lithostratigraphy. Bull. Geol. Soc. Finland 75(1–2), 29–50.
III:
Lauri, L.S., Rämö, O.T., Huhma, H., Mänttäri, I., Räsänen, J. Petrogenesis of
silicic magmatism related to ~2.44 Ga rifting of the Archean crust in Koillismaa,
eastern Finland. Lithos (submitted).
L. S. Lauri’s contribution to I included everything except U–Pb dating, which was done
by the co-author. In II, L. S. Lauri was mainly responsible for the field geology, petrography,
and geochemistry of the Sirniö Group, while the co-authors handled lithostratigraphy. In III,
L. S. Lauri’s contribution included field geology, whole-rock geochemistry, a part of the Nd
isotope analyses, and petrological modeling.
1. Introduction
Continental extension is often characterized by an anorogenic magmatic association
comprising layered mafic intrusions and dike swarms as well as silicic plutonic and volcanic
rocks. A world-wide phase of anorogenic, extension-related magmatism occurred at the
Archean-Proterozoic transition, 2.5–2.4 Ga ago. At this time, many extensive mafic complexes
were emplaced in Archean terranes, including (1) the 2.47–2.45 Ga Hearst-Matachewan dikes
in North America (Heaman, 1997), (2) the 2.42 Ga Jimberlana intrusion of Australia (Parker
et al., 1987), (3) the 2.42 Ga Scourie dikes of Scotland (Heaman and Tarney, 1989), (4) the
2.42 Ga high-Mg tholeiite dikes of Vestfold Block, Antarctica (Collerson and Sheraton, 1986),
and (5) the 2.50–2.44 Ga layered mafic igneous complexes of northern Fennoscandia (Alapieti
et al., 1990). The 2.5–2.4 Ga event has been interpreted to indicate rifting of a late Archean
supercontinent (e.g., Windley, 1995; Heaman, 1997), although it is not clear how many of the
above mentioned Archean fragments were attached to each other at this time. Paleomagnetic
evidence suggests that at least the Superior and Wyoming cratons of Laurentia and the Karelian
craton of Fennoscandia were joined (e.g., Pesonen et al., 2003).
This work concentrates on the silicic rocks associated with early Paleoproterozoic rifting
of the Archean nucleus of the Fennoscandian shield. The rocks include a quartz alkali feldspar
syenite pluton and a group of felsic and intermediate volcanic rocks that are closely associated
with a ~2.44 Ga mafic layered intrusion. The aim of the study was to investigate the previously
little known silicic part of the early Paleoproterozoic magmatic system in Koillismaa, using
field geology, petrography, lithostratigraphy, whole-rock geochemistry, and Nd isotope
geochemistry. The origin of the granophyric rocks associated with the Koillismaa layered
igneous complex was studied in detail. In the course of the study, Nd isotope data and new
U–Pb ages were also acquired for the Archean crust that hosts the early Paleoproterozoic
rocks. Based on these data, the origin of the ~2.44 Ga silicic rocks of Koillismaa was discussed
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in the papers I, II, and III. A comparison was also made to other anorogenic magmatic systems
that contain layered mafic intrusions and granophyric rocks and to known occurrences of
early Paleoproterozoic volcanic and plutonic rocks. The main results of the study are
summarized in this synopsis.
2. Previous studies
Geological mapping of the Koillismaa area started already at the beginning of the 20th
century and the work was completed in the 1950’s, when geological map sheets with
explanations on the 1:400 000 scale were published from the area (Enkovaara et al., 1953;
Matisto, 1958). In the late 1950’s, mining companies began to take interest in the ore potential
of the “Syöte-type gabbros” and, in the 1960’s, extensive geological mapping, geophysical
surveys, diamond drilling, and pilot plant concentration tests on disseminated sulphide
occurrences were carried out by Outokumpu Co. Vanadium was quarried from an open pit
mine (Mustavaara) in one of the layered intrusions in 1976–1985 by Rautaruukki Co. In the
1970’s, the Koillismaa Project of the University of Oulu, focusing mainly on the study of the
basic magmatism in the area, produced several academic dissertations (see Piirainen et al.,
1978). Alapieti (1982) collected the information produced by the Koillismaa Project into a
Ph.D. thesis that summarized the structural, mineralogical, and geochemical aspects of the
Koillismaa layered igneous complex. Two new geological map sheets on the 1:100 000 scale
were also published from the area by the Geological Survey of Finland (GTK) (Honkamo,
1979; Lahti and Honkamo, 1980). In the 1990’s, M.Sc. theses were compiled at the University
of Oulu (Karinen, 1998) and the University of Helsinki (Landén, 1999), the latter in close
collaboration with the GTK, within the framework of a research project started in Koillismaa
in 1996.
So far, most of the publications from the Koillismaa area have dealt with the Koillismaa
layered igneous complex and its relation to other ~2.44 Ga layered intrusions in the northern
Fennoscandian shield (e.g., Alapieti and Piirainen, 1984; Piispanen and Tarkian, 1984; Alapieti
and Lahtinen, 1989; Alapieti et al., 1990; Saini-Eidukat et al., 1997; Alapieti and Lahtinen,
2002). The works of Lauerma (1982) and Kontinen et al. (1992) include some samples from
the Archean rocks of the Koillismaa area and present some isotope data on these samples.
3. Regional geology
The Koillismaa area (Fig. 1) is a part of the Archean Karelian province of the Fennoscandian
shield (Korsman et al., 1997; Koistinen et al., 2001). The oldest rocks in the area are Archean
ortho- and paragneisses and small granite bodies with ages around 3.0–2.7 Ga (Gaál and
Gorbatschev, 1987; III). At the beginning of the Proterozoic eon, the Archean craton
experienced intermittent rifting that began at ~2.50 Ga, resulting in widespread volcanism
and plutonism (e.g., Huhma et al., 1990; I; II). In the Koillismaa area, the onset of rifting at
~2.44 Ga produced a sequence of volcanic rocks (II), the mafic Koillismaa layered igneous
complex (Alapieti, 1982), and the silicic Kynsijärvi quartz alkali feldspar syenite pluton (I).
At the same time, a similar rock suite consisting of the Oulanka layered igneous complex
(e.g., Lavrov, 1979), the Sumian volcanic rocks, and the Nuorunen granite (Buyko et al.,
1995) was formed in the adjoining Oulanka area in present northwestern Russia (Fig. 1). The
extension continued sporadically for the next several hundred million years and a large amount
of mafic magma transected the fractured bedrock, forming numerous diabase dike swarms
(Vuollo, 1994). However, the overall extension never proceeded to sea-floor spreading and
was halted at an intracratonic rift or aulacogen stage. Volcanic activity and sedimentation
7
N
26°E
Barents Sea
Norway
Kola
9
11
7
10
lo
Be
8
cle .5° N
Arctic cir 66
m
ia
or
Sweden
n
Finland
6
B
4
3
12
Gulf of Bothnia
A
Russia
White Sea
5
Fig. 2
Karelian
C
0
100 km
Archean rocks
Proterozoic plutonic rocks
Proterozoic metasedimentary and metavolcanic rocks
Proterozoic Lapland granulite complex
Mesoproterozoic sedimentary rocks (1.4-1.3 Ga)
Caledonides
Paleozoic alkaline intrusions
terrane boundary
national boundary
2.50-2.44 Ga mafic layered intrusions
1. Kukkola-Tornio
2. Kemi
3. Penikat
4. Portimo
5. Koillismaa
6. Oulanka
7. Koitelainen
8. Akanvaara
9. Monchegorsk
10. Imandra
11. Fedorova and Panski Tundra
2.44 Ga silicic intrusions:
A. Kynsijärvi quartz alkali feldspar syenite
B. Nuorunen granite
C. Kuhmo granites
Fig. 1. General bedrock map of the northern Fennoscandian shield and locations of the early Paleoproterozoic
mafic and silicic intrusions (modified after Alapieti and Lahtinen, 1989; Korsman et al., 1997; Koistinen et
al., 2001).
continued in the N-NW-side of the Koillismaa area, forming the ~2.3–2.0 Ga supracrustal
Kuusamo belt (Silvennoinen, 1991).
3.1. The Kynsijärvi quartz alkali feldspar syenite
The Kynsijärvi quartz alkali feldspar syenite pluton (I) is a small (1 km by 5 km), EWelongated intrusion that was emplaced in the Archean gneiss complex near the Kuusijärvi
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and Lipeävaara blocks of the Koillismaa layered igneous complex (Fig. 2). The intrusion is
very poorly exposed and the size estimate is based on low altitude aeromagnetic maps and
diamond drilling that was carried out by the GTK in 1997. No contacts are exposed but both
Archean gneisses and gabbroic rocks were encountered in the drill cores close to the presumed
contact of the pluton.
3.2. Lithostratigraphy
The supracrustal rocks in the study area are divided into three groups, the oldest of which
probably predates the layered intrusion. The other two groups are slightly younger but the
age difference may be negligible. The lithostratigraphy (Fig. 3) is described in detail in II.
The oldest supracrustal sequence, the Sirniö Group, consists of two volcanic formations.
The Sirniövaara Formation is up to several hundred meters thick, fine-grained, and on outcrop
scale relatively homogeneous, and consists of plagioclase crystal tuff with a granophyric
groundmass. Some breccia layers are also found within the unit. The overlying Unijoki
Formation consists of several felsic and intermediate volcanic units that lack granophyric
texture. The internal stratigraphy of the Unijoki Formation cannot be investigated in detail as
Proterozoic
< 2.44 Ga supracrustal rocks
N
Kaukua
Unijoki Formation
Sirniövaara Formation
quartz alkali feldspar syenite
gabbro (age unknown)
Lipeävaara
Kynsijärvi
diabase dikes
Koillismaa layered igneous complex
anorthosite
magnetite gabbro
olivine gabbronorite
gabbronorite
microgabbronorite
marginal series
Kuusijärvi
Ab-Qtz-rock
Pyhitys
Archean
granite gneiss
Porttivaara
supracrustal rocks
granulite
Other
fault
Syöte
0
5
10 Kilometers
Fig. 2. General bedrock map of the western intrusion of the Koillismaa layered igneous complex and associated
rocks (modified after Alapieti, 1982; Räsänen et al., 2003).
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Lamminlehto Formation
siltstone member
Fig. 3. Lithostratigraphic column of the
Paleoproterozoic supracrustal rocks of
Koillismaa (II).
Hautavaara Group
quartzite member
the units are generally poorly
exposed. The Sirniövaara Formation
Pajuniemi Formation
can be found overlying all the blocks
Kuusijärvi Formation
of the Koillismaa layered igneous
Löytövaara member
complex, whereas the rocks of the
Unijoki Formation are observed in a
Kynsijärvi Group
few locations only in the study area
Kivivaara member
(Fig. 2).
Unikumpu Formation
A thick conglomerate begins the
Unijoki Formation
succession
in the Kynsijärvi Group
Sirniö Group
Sirniövaara Formation
that overlies the Sirniö Group. The
Koillismaa layered
Kynsijärvi Group that comprises the
igneous complex
Kynsijärvi quartz alkali
sedimentary Unikumpu Formation
feldspar syenite
and the volcanic Kuusijärvi
Archean
basement complex
Formation has been tentatively
correlated to the lowermost units in
the nearby Kuusamo belt (Juopperi, 1976; Karinen, 1998). The Unikumpu Formation
corresponds to the basal conglomerate of the Kuusamo belt. The Kuusijärvi Formation that
consists of the hyaloclastic breccias of the Kivivaara Member and the basaltic flows of the
Löytövaara member, resembles the Greenstone Formation I of the Kuusamo belt described
by Silvennoinen (1991) (cf., Karinen, 1998).
The youngest supracrustal unit in the study area is the Hautavaara Group that comprises
the conglomerates of the Pajuniemi Formation and the quartzites of the Lamminlehto
Formation. The Pajuniemi conglomerate contains abundant clasts from the underlying
Kuusijärvi Formation, but also some fragments of the granophyres of the Sirniövaara Formation
have been found (Karinen, 1998). The quartzites of the Lamminlehto Formation grade from
orthoquartzites to subarkosic quartzites. Pelitic interlayers are found in the upper parts of the
formation.
3.3. Sampling and analytical methods
The rocks of the Sirniö Group were sampled for whole-rock chemistry during the mapping
of the study area in 1998–2000. Grab samples were collected from the outcrops. Diamond
drilling was also carried out at several locations (II) and the drill cores were logged and
sampled. Most of the sampling was done by the author, additional samples were taken by T.
Karinen and J. Räsänen. The sampling of the Kynsijärvi quartz alkali feldspar syenite was
done mostly from drill cores recovered in 1997. A total of 216 whole-rock chemical samples
were analyzed. Of these, 118 were from the Sirniövaara Formation, 74 from the Unijoki
Formation (44 felsic and 30 intermediate volcanic rocks), 15 from the Kynsijärvi quartz
alkali feldspar syenite, 6 from the Archean rocks in Koillismaa, and 3 from the Nuorunen
granite near the Oulanka layered complex in Russia (Fig. 1; III). Polished thin sections were
prepared from most of the samples and petrographic and mineralogical determinations were
made from them (I, II). The whole-rock chemical analyses were made by the XRF method.
Selected samples were also analyzed for the REE, Sc, Th, U, and Y by the ICP-MS method.
The 29 samples analyzed for Nd isotope composition were chosen mostly on the basis of
whole-rock chemistry (III). For U–Pb geochronology, one sample was analyzed from the
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Kynsijärvi quartz alkali feldspar syenite (I) and six from the surrounding Archean rocks in
Koillismaa from which previous data was insufficient (III). Whole-rock chemical analyses
were made at the Geolaboratory of the GTK, the electron microprobe work was carried out at
the mineralogical laboratory of the GTK, and the U–Pb geochronological and Nd isotope
analyses were performed at the isotope laboratory of the GTK. The analytical methods are
decribed in detail in I, II and III.
4. Petrography
4.1. Kynsijärvi quartz alkali feldspar syenite
A detailed petrographic description and microprobe data on the Kynsijärvi quartz alkali
feldspar syenite is presented in I. The quartz alkali feldspar syenite is red, medium-grained,
and leucocratic. It is also isotropic, homogeneous, and undeformed with the exception of one
drill core that shows some brittle deformation. The dominant mineral is hypersolvus alkali
feldspar, the modal amount of which is 72.1 to 84.4 vol.%. Other major minerals are quartz
and ferro-edenite. Accessory minerals include magnetite, titanite, zircon, fluorite, and
subsolidus stilpnomelane. The feldspar is a subhedral to euhedral mesoperthite with welldeveloped intergranular albite rims (Fig. 4a). The orthoclase and albite components of the
feldspar have exsolved quite effectively so that the amount of Or-component in the orthoclase
is ~98% and the Ab-component in the albite is >99%. An-component is negligible.
Quartz was crystallized in two stages. The first generation is present as euhedral short
prismatic quartz. The second generation is found as anhedral crystals in the interstices of the
feldspar crystals. The subhedral ferro-edenite is also interstitial relative to the feldspar and is
commonly associated with magnetite and titanite. The calculated Fe/(Fe+Mg) of the ferroedenite is close to 0.9 (I). In the eastern parts of the pluton the rock is unaltered and undeformed,
whereas in the western parts brittle deformation is found and the feldspar crystals are fractured
and ferro-edenite has been altered to stilpnomelane.
4.2. Sirniö Group
4.2.1. Sirniövaara Formation
The Sirniövaara Formation (II) consists of fine-grained, gray rhyodacite that is relatively
homogeneous on outcrop scale. Weak to moderate schistosity can be observed in most outcrops
and small shear zones are occasionally present. Tension veins and joints filled with quartz,
epidote, chlorite, carbonate, and sulphides are also commonly found. The veins vary in width
from a few millimeters to several meters.
In thin section, the rhyodacite is microporphyritic with subhedral to euhedral plagioclase
phenocrysts in a fine-grained groundmass that often shows a granophyric intergrowth texture
between quartz and albite (Fig. 4b, c). The major rock forming minerals are plagioclase,
quartz and biotite. Accessory minerals include chlorite, K-feldspar, zircon, apatite, fluorite,
carbonate, epidote, clinozoisite, sericite, oxides, titanite, and sulphides. At least one generation
of schistosity is seen in most thin sections. Thin veinlets of quartz, epidote, and carbonate are
also commonly present in the more altered samples.
Plagioclase phenocrysts are blocky or elongated, up to 2 mm in length. They are twinned
according to albite, Carlsbad, Baveno or pericline laws, the two first being the most common.
Some of the crystals have been broken into several pieces. Most of the phenocrysts are albite
and some sericitisation and saussuritisation is commonly observed.
11
Fig. 4. Photomicrographs of (a) the Kynsijärvi quartz alkali feldspar syenite, crossed polars. Afs, alkali
feldspar; Q1, primary quartz; Q2, second generation quartz; Ab, albite; Hbl, ferro-edenite; F, fluorite; Mgt,
magnetite; S, stilpnomelane (I), (b) and (c) euhedral plagioclase phenocrysts in a groundmass that shows a
granophyric texture of irregular radiating fringe type, samples LSL-98-18 (b) and R333 17.45 m (c), crossed
polars, (d) anhedral quartz crystal with vermicular granophyric intergrowths of albite, sample LSL-00-65,
crossed polars, (e) felsic lava, sample LSL-98-112, crossed polars, (f) felsic tuffite, sample R336 56.90 m,
crossed polars.
Groundmass plagioclase is anhedral albite that typically forms granophyric intergrowths with
quartz. In most samples, the granophyric intergrowths form irregular radiating fringes around
the albitic phenocrysts and the granophyric albite shares crystallographic orientation with the
albite in the phenocrysts (Fig. 4b, c). In the more altered samples, a vermicular granophyric
intergrowth is seen at the margins of larger quartz patches (Fig. 4d). In deformed samples, the
granophyric intergrowth is replaced by a fine-grained groundmass of granoblastic albite and
quartz.
12
Brown biotite forms aggregates of small, randomly oriented crystals that commonly contain
small zircon inclusions surrounded by pleochroic haloes. Biotite compositions fall between
siderophyllite and annite and show a trend toward phlogopite in the altered samples. Fe/Mg
ratio varies between 0.5 and 0.9. Green, Fe-rich chlorite forms lamellae in the biotite crystals.
In the lower parts of the formation and in local shear zones, biotite is almost completely
replaced by chlorite.
The most common oxide mineral is euhedral to subhedral ilmenite, but magnetite is also
found. Some of the ilmenite crystals are skeletal. The oxides commonly have a very finegrained rim of secondary titanite. Titanite also fills the interstices of the skeletal ilmenites.
Apatite is found as long, thin, broken needles. Zircon occurs as small, euhedral, turbid crystals,
whereas fluorite is found as small, anhedral crystals. Sulphides are present as small, anhedral
to euhedral grains both disseminated through the rock and in quartz veinlets. Epidote and
clinozoisite are found as anhedral to euhedral crystals of varying size, epidote also in veins.
Anhedral carbonate also occurs as vein fillings and replaces other minerals, mostly plagioclase.
Sericite is found as small, subhedral to euhedral crystals oriented along the plains of schistosity.
4.2.2. Unijoki Formation
The rocks of the Unijoki Formation (II) form a heterogeneous group of felsic to intermediate
volcanic rocks that mostly consist of pyroclastic or epiclastic material. Some lavas are also
found, often containing fragments of coarse-grained plagioclase porphyry. Textures such as
accretionary lapilli and spherulites are present in some units.
The felsic varieties commonly have plagioclase phenocrysts in a fine-grained groundmass
from which the granophyric intergrowth texture is lacking (Fig. 4e, f). The groundmass mainly
consists of quartz, albite, and biotite. Schistosity is usually present and some samples also
show textures that to some degree resemble bedding and cross-bedding.
In the intermediate varieties, the major minerals are plagioclase, amphibole, epidote, and
quartz. Accessory minerals include carbonate, biotite, chlorite, titanite, hematite, scapolite,
and zircon. The intermediate rocks are interpreted to be porphyritic lavas that contain relics
of amphibole and plagioclase phenocrysts. The plagioclase is pervasively saussuritized and
often replaced by scapolite. The amphiboles commonly have a light green core of tremoliteactinolite and a darker green rim of hornblende. Twinning is seen in some of the amphibole
crystals.
4.3. Nuorunen granite
The description of the Nuorunen granite is based on Hackman and Wilkman (1929) and on
the three samples used in the Nd isotope work (III). The pluton consists of a somewhat
deformed pink, and medium- to coarse-grained granite. Associated with the main intrusion
are quartz porphyry dikes that resemble the granite in mineral composition and geochemistry.
The major minerals are microperthitic alkali feldspar, quartz, plagioclase, and amphibole.
Accessory minerals include zircon, fluorite, oxides, allanite, and biotite. Micrographic
intergrowths of quartz and alkali feldspar are common. The amphiboles are partly replaced
by chlorite and secondary minerals such as sericite, stilpnomelane, and carbonate are also
common.
4.4. Archean rocks
The main rock types in the Archean crust of Koillismaa are medium- to coarse-grained,
variably deformed gneisses and granitoids, although some small occurrences of greenstones
13
have also been found. Gneisses comprise both ortho- and paragneisses the latter of which
have been strongly migmatized. Amphibolite fragments are found enclosed in the gneisses in
some locations. The granitoids commonly occur as small, irregular bodies among the gneisses.
Major minerals in the samples used for isotope geology in III are quartz, plagioclase, and
alkali feldspar. The mafic minerals include biotite, hornblende, and in some cases
clinopyroxene and orthopyroxene. Magnetite, sericite, epidote, zircon, and monazite are
common as accessory phases.
5. U–Pb geochronology
5.1. Kynsijärvi quartz alkali feldspar syenite
Five zircon fractions were analyzed from the Kynsijärvi quartz alkali feldspar syenite
(Fig. 5). Three of these were air-abraded. The zircon crystals were transparent and almost
colorless prisms with pyramidal edges. In BSE images they show typical magmatic zoning
with no evidence of inherited cores or metamorphic growths. The analytical results are
presented in I. The five-point discordia line calculated from the analyses interceps the concordia
curve at 2445±7 Ma and 415±70 Ma with a relatively high MSWD of 4.3. Ignoring the
strongly discordant point E the intercept ages are 2442±3 Ma and 341±47 Ma (MSWD = 1.8,
n = 4). The upper intercept is interpreted as a close approximation of the emplacement age of
the rock.
0.48
2500
TIMS U-Pb age data on zircons
A1585 Kynsijärvi quartz
alkali feldspar syenite
2400
B
0.44
206
Pb/
238
U
2300
2200
A
5.2. Archean rocks
0.40
2100
D
Fig. 5. Concordia diagram for the
five analyzed zircon fractions
from the Kynsijärvi quartz alkali
feldspar syenite. Ellipses denote
the 2σ errors in the variables (I).
C
Of the six samples
chosen for the U–Pb
0.36
geochronology from the
Intercepts (A,B,C,D) at
Archean
rocks
in
2441.9 ± 2.4 & 341 ± 47 Ma
E
MSWD = 1.8; n=4
Koillismaa, two were
0.32
granulite facies gneisses
6.5
7.5
8.5
9.5
10.5
and
four
were
207
235
Pb/ U
orthogneisses and granites
that, according to field
observations, represent the youngest rock types in the area. The details of the zircon and
monazite fractions analyzed from the samples are found in III. The zircon fractions were
mostly discordant and showed evidence of mixing of different age populations (Fig. 6).
Tentative zircon ages were acquired from only two samples. The Aholamminvaara granite
(sample A1656) yielded a five-point discordia line that intercepts the concordia curve at
2711±9 Ma and 270±450 Ma (MSWD = 2, n = 5). The Kaplaskumpu pyroxene tonalite
(sample A1661), a granulite facies gneiss, gave an upper intercept age of 2808±20 Ma (MSWD
= 3.9, n = 4). The four analyzed monazite fractions, all from different samples, gave concordant
ages close to 2695 Ma, regardless of the geological unit they were analyzed from. This age is
interpreted as the peak of the granulite facies metamorphism and, possibly, the emplacement
age of the granites.
14
0.56
A1656 Aholamminvaara granite
A1657 Harjavaara granite
A1642 Raatekalliot granite
A1644 Hanhimännikkö gneiss
0.54
2800
2600
0.50
2500
2700
A1657E monazite
206
A1656E
A1656B
A1657B
A1657A
A1657D
A1656H
0.48
Pb/238U
A1657C
A1656CDFG
2600
A1656A monazite
206
Pb/238U
0.52
0.46
2400
A1656 Intercepts at
270±450 & 2711±9 Ma
MSWD = 2, n = 5
2200
A1644A
A1644C
A1642B
0.34
11
13
12
207
14
0.30
15
A1644 Reference line
650 & 2650 Ma
A1644B
a)
10
A1642C
A1642A
2300
0.42
0.38
2500
0.44
b)
7
9
13
11
Pb/235U
207
15
Pb/235U
0.57
0.57
A1661 Kaplaskumpu pyroxene tonalite
A1662 Matovaara tonalite
2820
0.55
2820
0.55
2780
Pb/238U
A1661D monazite
A1661B
A1662B
A1662C
2740
0.53
A1662D
2700
A1662E monazite
206
2700
2780
A1661E
A1661A
A1661C
2740
0.53
206
Pb/238U
2800
2700
A1662A
0.51
0.51
Intercepts at
702±850 & 2808±20 Ma
MSWD = 3.9, n = 4
0.49
c)
0.49
d)
0.47
0.47
13.0
13.4
13.8
14.6
14.2
207
15.0
15.4
235
Pb/ U
13.0
13.4
13.8
14.2
207
14.6
15.0
15.4
Pb/235U
Fig. 6. Concordia diagrams for analysed zircon and monazite fractions from the Archean samples: (a) A1656
Aholamminvaara granite (black ellipses) and A1657 Harjavaara granite (open ellipses), (b) A1642 Raatekalliot
granite and A1644 Hanhimännikkö gneiss, (c) A1661 Kaplaskumpu pyroxene tonalite, (d) A1662 Matovaara
tonalite. Monazite fractions are marked with gray ellipses in a), c) and d). Ellipses denote the 2σ errors in
the variables (III).
6. Geochemistry
Whole-rock chemical description of the Kynsijärvi quartz alkali feldspar syenite is found
in I and the geochemistry of the Sirniö Group is described in detail in II. This short compilation
of the geochemistry of all the studied rock types is based on I and II, as well as (for the
samples chosen for isotope geology) III. Geochemical description of the Nuorunen granite
and the Archean samples utilized in the Nd isotope studies is also found in III.
6.1. Whole-rock geochemistry
6.1.1. Kynsijärvi quartz alkali feldspar syenite
The Kynsijärvi quartz alkali feldspar syenite pluton is geochemically quite homogeneous
and shows A-type characteristics in the sense of Loiselle and Wones (1979), and Eby (1990).
Its SiO2 varies between 70.1 and 71.7 wt.% and the total amount of alkalies is higher than 9.9
wt.% with Na/K > 1 in all samples. The rock is metaluminous with an average A/CNK value
of 0.96 (Fig. 7a). Fe/Mg is high [FeO*/(FeO* + MgO) > 0.9; Fig. 7b] and the amounts of
MgO, CaO, and TiO2 are quite low (~0.1, ~0.6, and ~0.2 wt.%, respectively).
Trace element data show high average values of Ba, Zr, Zn, Ga, and Nb and low values of
Sc, V, Ni, and Co, most of which were under detection limit (I). Primitive mantle-normalized
diagrams show deep negative anomalies of Sr, P, and Ti (Fig. 8a). Light REE are enriched
compared to heavy REE with average (La/Yb)N of 12.7 (Fig. 9a). A negative Eu-anomaly is
also present and Eu/Eu* averages at 0.28.
6.1.2. Sirniö Group
The rocks of the Sirniö Group can be divided into three geochemical groups. The Sirniövaara
Formation is relatively homogeneous when compared to the rocks of the Unijoki Formation;
15
2.0
a)
Peraluminous
A/CNK
Fig. 7. Composition of the Archean granitoids
and the 2.44 Ga rocks of Koillismaa plotted in
(a) A/CNK vs. SiO 2 , and (b) FeO*/
(FeO*+MgO) vs. SiO 2 diagrams. The line
separating the ferroan and magnesian fields in
b) is from Frost et al. (2001).
1.0
FeO*/(FeO*+MgO)
Metaluminous
the latter can be roughly divided into
Sirniövaara Formation
a “felsic group” with SiO2 > 63 wt.%
Unijoki Formation felsic
Unijoki Formation intermediate
and an “intermediate group” with
Kynsijärvi quartz alkali feldspar syenite
Nuorunen granite
SiO2 < 63 wt.%. The Sirniö Group as
Archean granitoids
0.0
a whole seems to follow a subalkaline
1.0
b)
differentiation trend (Fig. 10; cf.
Winchester and Floyd, 1977). The
0.9
rocks
also
show
A-type
ferroan ian
s
characteristics, though not as strongly
magne
0.8
as the Kynsijärvi quartz alkali
feldspar syenite.
0.7
The Sirniövaara Formation is
composed of rhyodacitic rocks with
0.6
SiO2 values ranging from 64.4 to 70.4
wt.%. Many of the major elements
0.5
show quite a variation but, throughout
50
70
80
60
SiO2
the unit, only TiO2 (0.8–1.0 wt.%) and
Al2O3 (10.6–13.4 wt.%) show trends that may indicate fractional crystallization (II). The
rocks are subalkaline and the less altered samples are metaluminous with A/CNK < 1.1 (Fig.
7a). Fe/Mg is high in the less altered samples [FeO*/(FeO* + MgO) ~0.9; Fig. 7b]. P2O5
values concentrate around 0.3 wt.% and MnO values around 0.1 wt.%.
Typical features in the trace element composition of the Sirniövaara rhyodacite are, e.g.,
relatively high average Zr value of ~270 ppm and low values of compatible elements such as
Cr and Ni (under the detection limit in most samples; II). Nb shows a clear negative anomaly
in the primitive mantle-normalized diagrams (Fig. 8b). The Ga/Al is high (10000*Ga/Al
between 3 and 6). Rare earth element patterns show enrichment of light REE compared to
heavy REE with average (La/Yb)N of 7.4 (Fig. 9b). Most samples show a negative Eu anomaly
(Eu/Eu* between 0.45 and 0.85).
The SiO2 content ranges from 63.1 to 79.1 wt.% in the felsic group of the Unijoki Formation.
The rocks are subalkaline and metaluminous to peraluminous with A/CNK between 0.7 and
1.9 (Fig. 7a). Fe/Mg is generally high [average FeO*/(FeO* + MgO) ~0.9] except for a few
samples with values close to 0.6 (Fig. 7b). Trace element patterns are quite similar to the
Sirniövaara Formation with an average Zr content of 276 ppm and a negative Nb anomaly in
a primitive mantle-normalized diagram (Fig. 8c). REE patterns (Fig. 9c) show enrichment of
light REE over heavy REE [average (La/Yb)N ~9.2] and a negative Eu anomaly (Eu/Eu*
between 0.52 and 0.85).
In the intermediate group of the Unijoki Formation, the SiO2 content ranges from 50.7 to
60.9 wt.%. Fe/Mg is lower than in the felsic group [FeO*/(FeO* + MgO) between 0.55 and
0.86; Fig. 7b]. Compatible trace elements (Cr, V, Ni, Zn) show higher values than in the felsic
group, while the Zr values are lower, commonly close to 100 ppm. These rocks also show a
negative Nb anomaly (Fig. 8c). The intermediate group shows some enrichment of light REE
with average (La/Yb)N of ~9.0 (Fig. 9c). A small negative Eu anomaly is present (Eu/Eu*
between 0.63 and 1.00).
16
1000
Sample/Primitive mantle
100
10
1
1000
Sample/Primitive mantle
Kynsijärvi quartz
alkali feldspar syenite
Nuorunen granite
a)
0.1
10
1
0.1
c)
Unijoki Formation
Felsic
Intermediate
100
10
1
Rb Ba Th U Nb K La Ce Pb Pr Sr P Nd SmZr Eu Ti Gd Tb Dy Y Ho Er Tm YbLu
1000
Sample/Primitive mantle
Sample/Primitive mantle
Sirniövaara Formation
100
Rb Ba Th U Nb K La Ce Pb Pr Sr P Nd SmZr Eu Ti Gd Tb Dy Y Ho Er Tm YbLu
1000
b)
d)
Archean granitoids
100
10
1
under detection
limit
0.1
0.1
Rb Ba Th U Nb K La Ce Pb Pr Sr P Nd SmZr Eu Ti Gd Tb Dy Y Ho Er Tm YbLu
Rb Ba Th U Nb K La Ce Pb Pr Sr P Nd SmZr Eu Ti Gd Tb Dy Y Ho Er Tm YbLu
Fig. 8. Primitive mantlenormalized trace element
diagrams for (a) the
Kynsijärvi quartz alkali
feldspar syenite (gray
field = all samples) and
the Nuorunen granite,
(b) the Sirniövaara
Formation (gray field =
all samples), (c) the
felsic (clear field = all
samples) and intermediate rocks (gray field
= all samples) of the
Unijoki Formation, and
(d) the Archean rocks.
The samples chosen for
the diagram were used in
Nd isotope determinations (III). Normalizing
values after Sun and
McDonough (1989).
6.1.3. Nuorunen granite
The Nuorunen granite is characterized by SiO2 contents of 75.1–75.7 wt.%. Like the
Kynsijärvi pluton, it shows A-type geochemical characteristics. The pluton is subalkaline
and metaluminous with A/CNK ~0.99 (Fig. 7a). Fe/Mg is quite high [average FeO*/(FeO* +
MgO) ~0.9; Fig. 7b]. Trace element data show Zr values close to 300 ppm and a small negative
Nb anomaly in the primitive mantle-normalized diagrams (Fig. 8a). Ga/Al is high (10000*Ga/
Al > 3) and (La/Yb)N is ~19.0 in the granites of the pluton and 9.4 in an associated quartz
porphyry dike (Fig. 9a). The negative Eu anomaly varies from 0.32 in the granites of the
pluton to 0.13 in the dike.
1000
Sample/C1 Chondrite
a)
Kynsijärvi quarz alkali
feldspar syenite
Nuorunen granite
100
10
Sample/C1 Chondrite
1000
100
10
c)
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Unijoki Formation
Felsic
Intermediate
100
10
1
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
1000
d)
Sample/C1 Chondrite
La Ce Pr Nd
Sample/C1 Chondrite
Sirniövaara Formation
1
1
1000
b)
Archean granitoids
100
10
1
heavy REE
under detection limit
0.1
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 9. C1 chondrite-normalized REE diagrams for (a) the Kynsijärvi quartz alkali feldspar syenite (gray
field = all samples) and the Nuorunen granite, (b) the Sirniövaara Formation (gray field = all samples), (c)
the felsic (clear field = all samples) and intermediate rocks (gray field = all samples) of the Unijoki Formation,
and (d) the Archean rocks. The samples chosen for the diagram were used in Nd isotope determinations
(III). Normalizing values after Sun and McDonough (1989).
17
80
Nuorunen granite
Rhyolite
Com/Pan
SiO2
70
RhyodaciteDacite
Kynsijärvi quartz alkali
feldspar syenite
Trachyte
60
Andesite
TrAn
Fig. 10. The Sirniö Group follows a subalkaline
differentiation trend in the classification diagram
of Winchester and Floyd (1977). The Kynsijärvi
quartz alkali feldspar syenite and the Nuorunen
granite are included for comparison. Symbols as
in Fig. 7.
6.1.4. Archean rocks
Phonolite
50
The Archean rocks form a
heterogeneous
geochemical group, in
Ab
Bas-Trach-Neph
which the SiO2 content varies from 65
40
to 75 wt.%. The samples with SiO2 <69
.001
0.01
0.1
1
10
wt.% are metaluminous whereas those
Zr/TiO2*0.0001
with a higher SiO 2 content are
peraluminous (Fig. 7a). Some common features are, e.g., the generally low Fe/Mg ratio that
places the Archean rocks in the magnesian field of Frost et al. (2001); only one granitic
sample (A1657) plots into the ferroan field (Fig. 7b). The Archean rocks also show a deep
negative Nb anomaly (Fig. 8d), and strong enrichment of light REE compared to heavy REE
(Fig. 9d). The granitic samples A1656 and A1657 have heavy REE contents under detection
limit. Eu anomaly is very small in most samples.
Sub-Ab
6.2. Nd isotope geochemistry
6.2.1. Kynsijärvi quartz alkali feldspar syenite
Four samples from the Kynsijärvi quartz alkali feldspar syenite were analyzed for Nd
isotopes (III). The concentrations of Sm (~6.8 ppm) and Nd (~41.0 ppm), and the 147Sm/
144
Nd ratio (~0.10) are quite uniform in three of the samples, whereas the fourth sample
showed markedly lower concentrations of Sm (3.6 ppm) and Nd (16.7 ppm) and a higher
147
Sm/144Nd (~0.13). The εNd value of –4.8 of three samples is considered as representative of
initial composition of the pluton and the time-integrated evolution of these samples (TDM
~3000 Ma) falls close to the evolution path of the local Archean crust (Fig. 11). In the fourth
sample, the initial εNd value (–7.5) and the TDM (3433 Ma) were clearly anomalous, probably
because of subsequent metamorphic disturbance.
6.2.2. Sirniö Group
Of the 14 samples analyzed for Nd isotopes from the Sirniö Group, nine were from the
Sirniövaara Formation, three from the felsic unit, and two from the intermediate unit of the
Unijoki Formation (III). The Sm concentrations vary from 6.1 to 8.8 ppm in the felsic samples
(Sirniövaara and Unijoki Formations) and are around 3.5 ppm in the intermediate samples.
The corresponding Nd concentrations are 31.1–46.5 ppm in the felsic rocks and 16.0 ppm in
the intermediate rocks and the 147Sm/144Nd ratios are 0.12 and 0.13, respectively. The precise
age of the volcanic rocks is unknown but the εNd values were calculated at 2440 Ma. Three
U–Pb zircon analyses from an old sample from the Sirniövaara Formation (GTK, unpubl.)
scatter randomly, but plot on the concordia diagram within the field of samples from the
Koillismaa layered igneous complex (Alapieti, 1982) and are thus not inconsistent with a
Paleoproterozoic, possibly 2.5–2.4 Ga age (H. Huhma, pers. comm., 2004). The rocks of the
Sirniö Group show average εNd values of –1.9 (felsic rocks) and –2.5 (intermediate rocks)
18
Fig. 11. εNd vs. age diagram
for the Archean granitoids
and the 2.44 Ga silicic rocks
of Koillismaa. DM is the
evolution of the depleted
mantle by DePaolo (1981a).
CHUR
denotes
the
chondritic uniform reservoir
(DePaolo and Wasserburg,
1976) (III).
and TDM ages between
2800 and 2950 Ma
(Fig. 11). One felsic
sample from the
Unijoki Formation and
two samples from the
Sirniövaara Formation
show
anomalous
values compared to the
other samples.
3
DM (DePaolo, 1981)
A1644
CHUR
0
Koillismaa and Oulanka
layered intrusions
e
Archean granitoids
calculated at 2700 Ma
Archean gneisses and
granulites
calculated at 2800 Ma
Nd
-3
LSL-00-83
Ev
in oluti
Ko on
illi of
sm the
aa
Ar
che
an
sam
ple
s
368-TTK-00
disturbed samples
LSL-00-65
-6
Kynsi
Sirniövaara Formation
Unijoki Formation:
Felsic
Intermediate
Kynsijärvi quartz
alkali feldspar syenite
Nuorunen granite
Archean granitoids
2900
2800
-9
2700
2600
2500
2400
-12
2300
Age/(Ma)
6.2.3. Nuorunen granite
The three samples from the Nuorunen granite have quite high elemental concentrations of
Sm (9–10 ppm) and Nd (50–67 ppm) and the 147Sm/144Nd ratio is relatively low, close to 0.1.
The average εNd (at 2440 Ma) value is –1.8 and TDM age is ~2750 Ma (Fig. 11). These values
differ from the Archean crust of Koillismaa but may reflect the (presumably different) Nd
isotope composition of the local Archean crust in Oulanka area (III).
6.2.4. Archean rocks
The Archean rocks show varying contents of Sm (between 0.9 and 6.6 ppm) and Nd
(between 8.7 and 64.2 ppm) and 147Sm/144Nd ratios between 0.059 and 0.121. The εNd (at 2700
Ma) values of most samples range from –3.8 to –1.3 and TDM ages are between 2900 and 3100
Ma (Fig. 11). The one sample with a markedly higher εNd (at 2700 Ma) value of +0.9 and
lower TDM age of 2800 Ma was taken close to the Oulanka complex and Nuorunen granite.
6.3. Post-crystallization disturbance
The rocks of the Sirniö Group were metamorphosed in at least greenschist facies conditions,
possibly clearly after the 2.44 Ga event. In the lower parts of the Sirniövaara Formation, the
degree of metamorphism may have reached upper amphibolite facies during the emplacement
of the Koillismaa layered igneous complex. The vermicular granophyre intergrowths and
strong alteration of biotite to chlorite are found in those samples that also show geochemical
evidence of hydrothermal alteration (II). The alkalies seem to have been mobile in the
Sirniövaara Formation, as demonstrated by the varying contents of Na2O (2.5–5.0 wt.%) and
K2O (0.2–4.0 wt.%). Higher than average values of MgO and CaO are also observed in the
samples that show evidence of alteration in thin section. The LILE such as Rb (15–134 ppm),
Sr (14–297 ppm), and Ba (81–1087 ppm) seem to have followed the alkalies and also show
scattered values (Fig. 8b). In general, the REE contents appear to be little affected by the
19
metamorphic processes, but Nd isotope data indicate that some of the samples (LSL-00-65,
LSL-00-83; Fig. 11) have experienced disturbance of the magmatic Sm–Nd system (III).
Evidence of alteration is also seen in the rocks of the Unijoki Formation. The highest SiO2
values in the felsic group are probably caused by secondary silicification. The alkalies have
probably also been mobile, as they show scattered values in both felsic and intermediate
rocks. The alteration of plagioclase to scapolite in the intermediate group also suggests the
influence of a Na-bearing fluid. The REE seem relatively unaffected also in the rocks of the
Unijoki Formation, but at least one felsic sample (368-TTK-00; Fig. 11) shows evidence of a
disturbed Sm–Nd system (III).
The only petrographic evidence of post-crystallization disturbance in the Kynsijärvi quartz
alkali feldspar syenite is the replacement of ferro-edenite by stilpnomelane and the anomalous
Nd isotope systematics of sample Kynsi (Fig. 11). The anomalous values of εNd (–7.5) and
TDM (3433 Ma) are probably the result of Sm/Nd fractionation at ~1.8–1.9 Ga (III). The
appearance of stilpnomelane may be a subsolidus phenomenon associated with the formation
of the intergranular albite rims – this probably happened during the cooling of the pluton and
is unrelated to the process that caused the disturbance of the Sm–Nd system.
6.4. Geochemical modeling
6.4.1. Boundary conditions
Geochemical models outlined in III are expanded here to include assessment of the origin
of the involved mafic magmas in the subcontinental mantle. The modeling was focused on
the Sirniövaara Formation and
the Kynsijärvi quartz alkali Table 1. Source rock compositions used in the partial
feldspar syenite, as these are the melting modeling of the Archean crust in Koillismaa
Average lower crust
Pyroxene tonalite A1661
largest and geochemically most Mineral
Source modea Melt modeb
Source modec Melt moded
uniform units in the area. For
0.05
0.06
0.15
0.166
the generation of the Hornblende
Biotite
0.10
0.13
0.10
0.11
Paleoproterozoic silicic rocks, Plagioclase
0.29
0.32
0.40
0.20
0.007
0.02
0.02
0.06
at least three possible scenarios Alkali feldspar
Clinopyroxene
0.05
0.03
0.03
0.005
exist. Considering the large Orthopyroxene
0.29
0.16
0.035
0.001
0.17
0.24
0.25
0.45
amount of magma that formed Quartz
Oxide
0.04
0.03
0.013
0.001
the layered mafic intrusions at Zircon
0.0001
0.0005
0.0001
0.001
0.0005
0.001
0.001
0.005
~2.44 Ga it would be possible Apatite
0.0001
0.0005
(in terms of volume) to Allanite
Titanite
0.001
0.001
fractionate relatively small
amounts of silicic melts from Trace element abundances (C0, in ppm) and D and P values:
them. The surrounding Archean
C0
D
P
C0
D
P
6.0
1.23
1.31
3.0
1.35
1.39
crust could also have been a Nb
Zr
70
0.86
2.70
270
0.94
4.98
source for the silicic rocks; in La
11.0
1.01
2.07
61.1
0.66
0.69
12.7
0.53
1.20
41.73
0.60
0.82
this scenario the mafic magmas Nd
1.17
0.93
1.08
1.27
1.56
1.37
would have acted as the heat Eu
Yb
2.2
0.65
0.71
0.81
0.90
1.25
source. The third option is a
a
combined
assimilation- Source mode as the molecular normative composition of the average lower crust
of Taylor and McLennan (1985), trace element abundances from Taylor and
fractional crystallization (AFC) McLennan (1985, Table 4.4).
b
Melt modes from Rämö (1991).
process.
c
Source mode calculated by J. Paavola.
The Nd isotope composition dEstimated after Wyllie (1977).
of
the
Archean
and D denotes the bulk partition coefficient, P denotes the bulk partition coefficient of
the melting phases in non-modal melting.
20
Paleoproterozoic rocks in Koillismaa places initial constraints on the three models. The εNd
(at 2440 Ma) values of the Archean rocks in Koillismaa vary between –5 and –8.5 (III),
which indicates that the Sirniövaara Formation with average εNd (at 2440 Ma) value of –1.9
was probably not derived by partial melting of the local Archean crust. The Koillismaa layered
igneous complex displays εNd (at 2440 Ma) values between –1 and –2 (Rämö et al., 2004),
which in turn indicates that the Kynsijärvi pluton with an average εNd (at 2440 Ma) value of
–4.8 (III) was probably not derived by fractional crystallization of the mafic magma.
Geochemical modeling on the parental magma of the 2.44 Ga mafic layered intrusions
was done by Saini-Eidukat et al. (1997) who used trace elements and by Hanski et al. (2001)
who used Sm–Nd and Re–Os isotopes. Their results have been utilized in constructing the
fractional crystallization and AFC models. In the partial melting models, the source
compositions were limited to middle and lower crustal sources, an assumption based on the
A-type geochemistry of the Kynsijärvi pluton. The mineral-melt partition coefficients used
in the models are found in Appendix 1.
6.4.2. Partial melting
For partial melting modeling, trace element modal and non-modal batch melting and
fractional melting models were deviced. Batch melting is an approximation where the partial
melt is constantly reacting and equilibrating with the residue until the melt is able to escape
the site of melting (e.g., Allègre and Minster, 1978). The concentration of a trace element in
the melt, CL, is described by equation
(1)
CL = C0/[D0 + F(1–P)]
where C0 is the concentration of the element in the solid, D0 is the bulk partition coefficient of
the element in the original solid, F is the weight fraction of the melt produced, and P is the
bulk partition coefficient in the mineral assemblage that enters the melt. In modal melting,
the minerals enter the melt in their modal proportions and P is substituted by D0 in equation
(1) (Shaw, 1970).
In fractional melting, a small amount of liquid is produced and instantly isolated from the
source. The concentration of a trace element in the melt, CL, is expressed by equation
(2)
CL = [C0(1 – PF/D0)(1/P–1)]/D0
The variables are the same as in batch melting. In the case of modal melting, P is substituted
with D0 and the equation (2) is simplified to
(3)
CL = [C0(1 – F)(1/Do–1)]/D0
Batch melting is a commonly used model for the generation of granitic liquids that are
relatively viscous and may not have the ability to easily escape the site of melting. Fractional
melting is used for the intermediate and mafic liquids that are more mobile. A-type silicic
melts are relatively hot and less viscous than the other types of granitic melts so fractional
melting models were also constructed for the Kynsijärvi quartz alkali feldspar syenite.
Mineral parageneses and elemental concentrations of the average lower crust were taken
from Taylor and McLennan (1985) and Rämö (1991). The pyroxene tonalite sample A1661
from Koillismaa is considered to represent the possible local middle crustal source (III). The
mode of the sample A1661 was calculated by J. Paavola (GTK). The mineral assemblage of
21
a)
sample/Primitive mantle
Fig. 12. Partial melting models with (a) lower
crust of Taylor and McLennan (1985) and (b)
pyroxene tonalite A1661 as the source
compositions. Primitive mantle-normalizing
values after Sun and McDonough (1989).
100
Kynsijärvi
Kynsijärvi
10
5%
Lower crust
10 %
30 %
50 % melting
5%
Lower crust
10 %
30 % melting
Lower crust batch melting modal
Lower crust batch melting non-modal
A1661 that enters the melt in nonmodal melting was approximated after
Wyllie (1977). The source and melt
modes for the sources and the bulk
partition coefficients are found in Table
Lower crust fractional melting
Lower crust fractional melting modal non-modal
1. The modeling involved Nb, Zr, La,
A1661 batch melting non-modal
A1661 batch melting modal
Nd, Eu, and Yb. These elements were b)
chosen for their relative immobility in
the post-crystallization processes that
have affected some of the samples.
Non-modal fractional melting of the
A1661 fractional melting non-modal
A1661 fractional melting modal
average lower crust produced
acceptable Zr, Nd, and Yb contents with
30% partial melting (Fig. 12a). The
other trace element contents and the
(La/Yb)N ratio of the modeled melts
indicate, however, that the Kynsijärvi
quartz alkali feldspar syenite was not
derived by partial melting of the
average lower crust. In particular, the high Nb content and the deep negative Eu anomaly of
the Kynsijärvi pluton are not reproduced by the models. Non-modal melting of the pyroxene
tonalite A1661 produces La, Zr, and Nd contents that are similar to the ones observed in the
Kynsijärvi pluton (Fig. 12b). However, this model also fails to produce the negative Eu
anomaly, and the Nb and Yb contents in the partial melts are too low in all models. Neither of
these sources seem to offer a viable alternative for the Kynsijärvi quartz alkali feldspar syenite.
sample/primitive mantle
1
100
Kynsijärvi
Kynsijärvi
10
5%
10 %
20 %
30 % melting
5%
Lower crust
10 %
30 %
50 % melting
Lower
crust
1
Nb
La
Nd
Zr
Eu
Yb
Nb
La
Nd
Zr
Eu
Yb
sample/Primitive mantle
1000
Kynsijärvi
100
Kynsijärvi
5%
10 %
30 %
50 % melting
1
1000
sample/Primitive mantle
A1661
A1661
10
5%
10 %
15 % melting
Kynsijärvi
100
Kynsijärvi
A1661
A1661
10
5%
10 %
30 %
50 % melting
1
Nb
La
Nd
Zr
5%
10 %
15 % melting
Eu
Yb
Nb
La
Nd
Zr
Eu
Yb
6.4.3. Fractional crystallization
To explore the fractionation hypothesis, both major element and trace element models
were deviced. Major element fractional crystallization modeling was carried out using a semiquantitative least-squares approximation approach (MAGFRAC; Morris, 1984). Although
the method is only a coarse approximation of the fractionating system, the modeling gives a
general trend of the evolution of the liquid and the mode of the fractionating assemblage that
can be used in the trace element models. The modeling was carried out in two stages. The
parental liquids were chosen utilizing previously published interpretations and modeling done
on the 2.44 Ga mafic layered intrusions and volcanic rocks in the Fennoscandian shield (Puchtel
et al., 1997; Saini-Eidukat et al., 1997; Hanski et al., 2001). In the first stage, the komatiite of
Lion Hills, Vetreny belt (Puchtel, 1997) was used as the parental liquid and the average
siliceous high-Mg basaltic (SHMB) parental magma of the 2.44 Ga layered mafic intrusions
(Saini-Eidukat et al., 1997) as the daughter. The second stage involved fractionation of the
average Sirniövaara Formation from the gabbroic (SHMB) daughter of the first stage. In line
with general igneous evolution trends, the Mg/Fe and Ca/Na ratios of the fractionating minerals
were higher in the first stage (for details, see Appendix 1). The established major element
fractionates were then used in trace element modeling. Trace element behavior in fractional
crystallization can be described by equation
22
(4)
CL = C0F(D–1)
where CL is the concentration of element in the residual melt, C0 is the concentration of
element in the initial melt, F is the fraction of melt left, and D is the bulk partition coefficient
of the crystallizing mineral assemblage for element. This model is a modification of the
Rayleigh distillation law (Rayleigh, 1896) and equilibrium is assumed only between the surface
of the crystallizing minerals and the melt.
Major element modeling of stage one indicates that when 31% olivine (Fo86.2), 7%
clinopyroxene, 7% plagioclase (An76.3) and <2% oxide fractionate from the komatiitic parent,
the evolved liquid is close in composition to the average SHMB parental magma of SainiEidukat et al. (1997), with the sum of squared residuals value of 0.324. The amount of the
liquid remaining is 53% of the original. The estimated komatiitic parental liquid shows
Table 2. Fractional crystallization model for the Sirniövaara Formation.
STAGE 1
STAGE 2
Fractionating assemblage:
F
ΣR2
Olivine 30.7%
Clinopyroxene 7.2%
Plagioclase 6.9%
Oxide 1.9%
Orthopyroxene 43.3%
Clinopyroxene 15.2%
Plagioclase 33.0%
Ilmenite 0.3%
Magnetite -1.0%
53%
0.324
8%
1.209
Model liquid compositions:
Initial
Initial
Assumeda Calculated
SiO 2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
48.0
0.45
8.51
10.8
0.18
22.9
7.02
1.64
0.30
0.07
Daughter
Assumedb
Initial
Assumedb
Initial
Calculated
Daughter
Assumedc
54.1
0.31
11.6
8.66
0.18
13.5
8.41
1.93
1.28
0.04
54.1
0.31
11.6
8.66
0.18
13.5
8.41
1.93
1.28
0.04
54.2
0.31
11.6
8.66
0.17
13.4
8.35
1.64
0.23
0.02
69.9
0.97
12.3
7.91
0.09
0.83
1.51
4.02
2.13
0.27
48.0
0.37
8.54
10.8
0.17
22.9
7.04
1.24
0.70
0.02
Trace elements:
D
Nb
Zr
La
Nd
Eu
Yb
0.06
0.03
0.15
0.05
0.03
0.12
STAGE 1
C0
1.325
40.3
5.1
6.31
0.48
1.02
CL
D
2.41
74.5
8.7
11.5
0.90
1.78
0.55
0.24
0.53
0.23
0.91
0.56
STAGE 2
C0
2.41
74.5
8.7
11.5
0.90
1.78
CL
7.51
508
28.5
80.6
1.13
5.41
a
Average of four samples (91103-91106) from Lion Hills, Vetreny Belt, Russia (Puchtel et al., 1997; Table 2).
Average SHMB parental magma (Saini-Eidukat et al., 1997; Table 3)
Sirniövaara Formation, average of 30 analyses (L. Lauri, unpubl.).
F = amount of residual liquid, ΣR2 = sum of squared residuals, FeO* = total Fe calculated as Fe2+, D = bulk partition
coefficient, C0 = concentration of element in the initial melt, CL = concentration of element in the residual liquid.
b
c
23
Fig. 13. Trace element fractional crystallization
model for the Sirniövaara Formation, (a) Stage 1:
fractionation of the average SHMB (Saini-Eidukat
et al., 1997) from the komatiite of Lion Hills, Vetreny
belt, Russia (Puchtel et al., 1997), (b) Stage 2:
fractionation of average Sirniövaara Formation (L.
Lauri, unpubl.) from the daughter high-Mg gabbro
of Stage 1. Primitive mantle-normalizing values after
Sun and McDonough (1989).
24
Sample/Primitive mantle
Sample/Primitive mantle
somewhat lower contents of TiO2, Na2O, and P2O5 and higher contents of K2O than the original
parent (Table 2). A parental composition with lower Ti could perhaps be used (e.g., SainiEidukat et al., 1997) to eliminate the TiO2 anomaly. The anomalies in Na, K, and P are explained
by the facts that mobility of alkalies is common in these rocks and the model did not involve
a phosphorus-bearing phase such as apatite. However, the model cannot be considered
quantitative, as the only mineral that can be expected to substantially fractionate from a
komatiitic liquid is olivine. However, it suggests that gabbroic magmas associated with the
mafic layered intrusions can be derived from a komatiitic parent via major fractionation of
olivine (cf. Hanski et al., 2001). In this model, the SiO2 content changes from 48 wt.% to 54
wt.%. A more realistic approach would be to involve several stages with different fractionating
minerals between the komatiite parent and the SHMB daughter. It is also questionable whether
the oxide that fractionates from a mafic magma at this early stage is magnetite, as required by
this model. However, the fractionation of chromite could not be modeled with MAGFRAC
and ilmenite was not compatible with the model.
Stage two involved fractionation of the average Sirniövaara Formation from the average
SHMB parental magma of Saini-Eidukat et al (1997). The fractionating assemblage was 33%
plagioclase (An63.1), 15% clinopyroxene, 43% orthopyroxene, and 0.3% ilmenite and the
amount of residual liquid was approximately 8%. The sum of the squared residuals was >1.2,
which indicates that the model is not very accurate. The estimated parental liquid shows
lower contents of MgO, CaO, Na2O, K2O, and P2O5 than the original parent. Other weaknesses
of the model are that the amount of orthopyroxene fractionation is quite high compared to
that of plagioclase and the melt requires approximately 1% added magnetite to balance the Fe
content.
The mineral parageneses of stages one and two were used in trace element modeling that
involved basaltic partition coefficients in stage one and dacitic-rhyolitic partition coefficients
in stage two. Nb and La contents produced by fractionation of the komatiitic parental magma
in stage one correspond quite well to the
contents in the SHMB magma (Fig. 13a).
100
Parent (komatiite)
However, the REE fractionation in the
Stage 1, F=53%
Daughter (high-Mg
modeled melt is much smaller than in the
gabbro)
SHMB, as seen in the (La/Yb)N ratio (3.5
average SHMB
(Saini-Eidukat et
and 7.4, respectively). Zr contents are also
al., 1997)
10
higher in the modeled melt than in the
SHMB.
The daughter trace element contents of
stage one were fractionated further in stage
a)
two, now with dacitic-rhyolitic partition
1
coefficients. Nb, La, and Eu contents
Nb
La
Nd
Zr
Eu
Yb
comparable with those of the Sirniövaara
100
Stage 2, F=8%
Formation are produced by 92%
fractionation of the SHMB magma (Fig.
10
Parent (high-Mg gabbro of Stage 1)
Daughter (SiO2 = 69.9 wt.%)
1
b)
Nb
Sirniövaara Formation
La
Nd
Zr
Eu
Yb
13b). In the modeled melt, the negative Eu anomaly is deeper and the Nd, Zr, and Yb contents
are markedly higher than in the Sirniövaara Formation.
The fractional crystallization model is not inconsistent with the derivation of the Sirniövaara
Formation from the mantle-derived magmas that produced the 2.44 Ga layered intrusions but
the model is rather inaccurate. The amount of felsic magma derived by fractional crystallization
of mafic magma is also very small. The observed thickness of the Sirniövaara Formation is
some hundreds of meters, which implies that the amount of the mafic parental magma should
have corresponded to an overall thickness of ~10 km, if the rocks of the Sirniövaara Formation
were derived by fractional crystallization.
6.4.4. Combined assimilation-fractional crystallization
The principle of AFC modeling is based on Bowen (1928), who first proposed that wallrock
assimilation is driven by the latent heat of crystallization during fractional crystallization.
The equations that describe the concentration of a trace element in a melt relative to the
original magma composition during an AFC process were derived by DePaolo (1981b) and
Powell (1984). AFC process is described with equation
(5)
CL = C0f’+rCA(1–f’)/(r–1+D)
where CL is the concentration of the element in the residual liquid, C0 is the concentration of
the element in the initial liquid, CA is the concentration of the element in the assimilant, r is
the ratio of the assimilation rate to the fractional crystallization rate, D is the bulk partition
coefficient for the element and f’ is described by the relation
(6)
f’ = F–(r–1+D)/(r–1)
where F is the amount of residual liquid and r and D are as in (5).
Previous modeling done on the 2.44 Ga mafic layered intrusions (Saini-Eidukat et al.,
1997; Hanski et al., 2001) is in favor of an AFC model with a komatiitic depleted mantlederived melt and an Archean crustal contaminant for the generation of the mafic magmas.
These models were used as a basis for the AFC modeling that was carried out to examine the
role of Nd isotope composition and variation of trace element contents in the 2.44 Ga silicic
rocks of Koillismaa. Fractionation of the komatiitic magma [εNd (at 2440 Ma) +2.6] was
modeled with the pyroxene tonalite A1661 and the lower crust of Taylor and McLennan
(1985) as the assimilants. The lower crust was assumed to have an εNd(at 2440 Ma) value of
–5, which is the most radiogenic value measured from the Koillismaa Archean samples and is
considered to comply with an Archean intermediate to mafic lower crust characterized by a
relatively high time-integrated Sm/Nd ratio (cf. Rudnick and Fountain, 1995). The pyroxene
tonalite A1661 has an εNd(at 2440 Ma) value of –7.6 and is thus one of the least radiogenic
samples analyzed from the Archean rocks of Koillismaa. The rate of assimilation (r; DePaolo,
1981b) was varied between 0.1 and 0.7. Basaltic partition coefficients were used until the
amount of residual liquid was 50%, after which the partition coefficients were changed to
dacitic-rhyolitic.
Assimilation of the average lower crust of Taylor and McLennan (1985) produced the Nd
isotope composition and some of the trace element characteristics of the Sirniövaara Formation
with an assimilation rate of r = 0.5 (Fig. 14a). However, the model is inconsistent with the
derivation of either the parental magma of the Koillismaa layered igneous complex or the
Kynsijärvi quartz alkali feldspar syenite from the contaminated komatiite, as the εNd of the
25
epsilonNd
3
2
1
0
-1
-2
-3
-4
-5
-6
0.9
0.7
K
0.9
0.9
0.2
0.7
0.9
0.1
r=0.1
0.5
0.4
0.7
0.9
0.3
0.2
r=0.3
0.5
SHMB
0.3
SF
0.5
0.4
0.5
r=0.5
0.7
0.1
0.2
300
400
LC
600 0
500
20
0.9
epsilonNd
0.9
40
60
-8
SHMB
0.9
0.7
0.5
0.4 0.3
0.9
0.2
r = 0.1
0.1
SHMB
0.9
0.5
0.4
0.7
100
120
SF
0.3
0.2
KS
0.3
0.7
r = 0.5
0.5
0.4
400
600
0.9
0.1
r = 0.3
A1661
200
0.1
r = 0.1
0.7
0.5
0.7
b)
0.2
0.9
SF
0
80
0.7
0.5
0.4 0.3
0.5 0.4
-6
0.1
0.9
0.7
-4
r = 0.7
0.2
K
K
-2
0.1
Nd
4
0
r = 0.5
KS
Zr
2
0.1
0.2
0.3
LC
200
r = 0.3
SF
0.5
0.4
0.1
r=0.7
KS
100
0.2
0.3
0.2
0.3
0.1
r = 0.1
0.3
0.1
0.4
a)
0.2
0.5
0.4
0.7
SHMB
0.4
0.7
0
0.9
0.7
0.5
0.4 0.3
0.9
0.7
K
0.5
0.4 0.3
0.9
0.4
KS
0.3
0.2
0.5
r = 0.3
0.4
0.3
0.2
0.7
r = 0.7
800
Zr
r = 0.5
r = 0.7
A1661
0.3
1000 0
20
40
60
80
100
Nd
Fig.14. The evolution of Zr and Nd concentrations in an AFC model with (a) the lower crust of Taylor and McLennan
(1985) and (b) the pyroxene tonalite A1661 as assimilant. K = average of four samples of the komatiite of Lion
Hills, Vetreny belt (Puchtel et al., 1997), SHMB = average parental magma of the 2.44 Ga layered intrusions (SainiEidukat et al., 1997), SF = Sirniövaara Formation [trace element data average of 30 analyses (L. Lauri, unpubl.), εNd
average of the unaltered samples in III], KS = Kynsijärvi quartz alkali feldspar syenite (I), LC = average lower
crust of Taylor and McLennan (1985).
modeled melt is still higher than in the 2.44 Ga plutonic rocks. The local pyroxene tonalite
A1661 as assimilant produced a better fit for all Paleoproterozoic rock types (Fig. 14b).
Consistently with the model of Hanski et al. (2001) for the Koitelainen and Akanvaara layered
intrusions, the Nd isotope composition of the Koillismaa layered igneous complex (~ –1.5;
Rämö et al., 2004) is derived by 10–15% assimilation of Archean crust with the εNd(at 2440)
value of –7.6. The trace element and the Nd isotope composition of the Sirniövaara Formation
and the Kynsijärvi quartz alkali feldspar syenite are produced by ~80–90% fractionation of
the mantle-derived magma with assimilation rates of r = 0.1 and r = 0.3, respectively (Fig.
14b). However, the model shows that both the Sirniövaara Formation and the Kynsijärvi
pluton would require an assimilant with a higher Nb content. One weakness of the model is
that the fractionating assemblages were the same that were used in the fractional crystallization
modeling and may thus not be altogether realistic. Also, the residual liquid value of 50% for
the modification of the partition coefficients was also chosen according to the fractional
crystallization model. Nevertheless, AFC seems to offer the best explanation for the generation
of the 2.44 Ga silicic rocks of Koillismaa.
7. Discussion
7.1. Geochronology of the Koillismaa area
The ages determined for the Archean rocks of the Fennoscandian shield range from rare
occurrences of >3.1 Ga rocks (Kröner et al., 1981; Paavola, 1986; Lobach-Zhuchenko et al.,
1993; Mutanen and Huhma, 2003) to the prominent group of 2.85 to 2.68 Ga old rocks (e.g.,
Vaasjoki et al., 1999 and references therein; Hölttä et al., 2000; Luukkonen, 2001; Evins et
al., 2002; Mänttäri and Hölttä, 2002). The Archean rocks in Koillismaa belong to the main
Neoarchean phase of crustal growth based on the age determinations in III. The gneisses, in
which the metamorphic degree may rise up to granulite facies conditions, are at least 2800
Ma old. The 2900 to 3100 Ma TDM ages of the Archean rocks suggest that even older rocks
26
might be found with more extensive sampling. The granitic magmatism and migmatization
took place at ~2700 Ma or even slightly later, if the monazite age of ~2695 Ma is taken to
represent the emplacement age of the granites. The monazites in the ~2800 Ma gneisses of
Koillismaa also record the 2695 Ma age, which is thought to coincide with the peak of granulite
facies metamorphism. In some parts of the Archean craton, granulite facies metamorphism
occurred at ~2.63 Ga (e.g., Hölttä et al., 2000; Mänttäri and Hölttä, 2002); however, the
Archean magmatic evolution of the Koillismaa area seems to have ceased at ~2.69 Ga. The
Archean K–Ar ages of hornblendes and biotites also indicate that these later processes did
not markedly affect the lithologic units of the area (Kontinen et al., 1992).
The Archean crust of Koillismaa experienced a period of cratonization that lasted until
~2.44 Ga, at which time upwelling of mantle beneath the Archean craton and resultant extension
started. The cause of the upwelling is debated – both a mantle plume model and passive
rifting controlled by pre-existing weaknesses of the lithosphere have been proposed (e.g.,
Amelin et al., 1995; Puchtel et al., 1997; Hanski et al., 2001). The Paleoproterozoic magmatism
in Koillismaa was connected to the rifting, the mafic magmas originating in the mantle and
the felsic magmas being produced as the mafic magmas interacted with the Archean lower
crust.
This tectonothermal event was quite short in duration, as shown by the U–Pb mineral data
published previously and in this work. The age of the Koillismaa layered igneous complex is
2436±5 Ma (Alapieti, 1982) and that of the Kynsijärvi quartz alkali feldspar syenite 2442±3
Ma (I). The Sirniövaara Formation is also presumably ~2.4–2.5 Ga old (see page 16 for
details). Furthermore, no substantial Archean inheritance has been detected in the zircon
population of the Paleoproterozoic rocks (Fig. 15).
The extensional tectonics continued intermittently in the Koillismaa area for the duration
of the Paleoproterozoic until ~2.0 Ga, resulting in the emplacement of several mafic dike
swarms (Vuollo, 1994; Vuollo et al., 2000). Also, the sill-like western intrusion of the
Koillismaa layered igneous complex was also broken into several fragments during the
extensional phase (Alapieti, 1982; Karinen, 1998; Karinen and Salmirinne, 2001).
Compressional tectonics prevailed in Koillismaa area at ~1.8–1.9 Ga, when the blocks of the
western intrusion of the Koillismaa complex were rotated to their present location and the
0.56
2800
TIMS U-Pb age data on
zircons and monazites
from Koillismaa, Finland
0.52
A1656 and
A1661
monazite
2600
Koillismaa layered igneous complex
2436±5 Ma (Alapieti, 1982)
0.48
206
Pb/
238
U
Proterozoic
Kynsijärvi quartz alkali feldspar syenite
2442±3 Ma (Lauri and Mänttäri, 2002)
2400
0.44
Archean
Soilu granodiorite (Lauerma, 1982)
filled ellipsoids: granite A1656
zircons 2711±9 Ma (MSWD = 2, n = 5),
monazite 2695±2 Ma
2200
0.40
open ellipsoids: pyroxene tonalite A1661
2808±20 Ma (MSWD = 3.9, n = 4),
monazite 2696±2 Ma
0.36
6
8
10
207
Pb/
12
235
14
U
Fig. 15. Concordia diagram for comparison of the Archean and Proterozoic rocks in Koillismaa (III).
27
hanging-wall supracrustal rocks (including the Sirniö Group) were sheared and folded
(Karinen, 1998; Karinen and Salmirinne, 2001; II). Fluid flow in the shear zones at this time
may also have affected some of the samples used in the Nd isotope determinations (III).
After the Paleoproterozoic, the Koillismaa area experienced a period of magmatic and tectonic
quiescence that was briefly broken at ~370 Ma, when the alkaline intrusion of Iivaara was
emplaced (Kramm et al., 1993).
7.2. The granophyre problem
Granophyre refers both to an irregular microscopic intergrowth texture between quartz
and alkali feldspar and to a porphyritic rock type displaying this kind of texture in the
groundmass (Bates and Jackson, 1987). The interpretations about the origin of the texture
vary from magmatic processes such as simultaneous crystallization of quartz and alkali feldspar
from a ternary eutectic granitic melt (e.g., Hughes, 1971) to metamorphic solid-state
recrystallization or devitrification (e.g., Walraven, 1985; McPhie et al., 1993). Granophyric
rocks form a part of many igneous suites, both mafic and felsic, and the depth of formation
varies from volcanic to plutonic (e.g., Hughes, 1971; Irvine, 1975; Pankhurst et al., 1978;
Agron and Bentor, 1981; Dickin and Exley, 1981; Walraven, 1985; Bloxam and Dirk, 1988;
Hirschmann, 1992; Lipman et al., 1997; Lowenstern et al., 1997; Mutanen, 1997). Layered
mafic intrusions are commonly associated with granophyric rocks. Besides Koillismaa,
extensive granophyres occur, e.g., in the Bushveld complex, South Africa (Walraven, 1985),
the Skaergaard intrusion, Greenland (Hirschmann, 1992), and the Koitelainen and the
Akanvaara intrusions, northern Finland (Mutanen, 1997). The genesis of the granophyres in
the mafic complexes is in many cases complicated and, in general, relatively little studied
compared to the efforts on the layered intrusions themselves. Interstitial granophyric material
is common in the upper cumulates of many mafic intrusions (e.g., Alapieti, 1982), but most
of the larger granophyre occurrences are considered to represent partial melts or recrystallized
parts of the roof and wall rocks of the intrusions (e.g., Smith, 1974; Irvine, 1975; Walraven,
1985).
Based on the data collected by the Koillismaa Project of the University of Oulu, Alapieti
(1982) interpreted the western intrusion of the Koillismaa layered igneous complex to be a
sill-like body that intruded a subhorizontal unconformity between the Archean basement and
overlying volcanic rocks the age of which is uncertain. He stated that the intrusion of the
mafic magmas resulted in the crystallization of the mafic layered sequence and the granophyre,
but he did not discuss the genesis of the latter in detail.
The Sirniö Group overlies the gabbroic and anorthositic cumulates of the Koillismaa layered
igneous complex. The contact is unexposed but no evidence of erosion of the upper cumulates
of the layered intrusion exists, contrary to what has been observed in the Penikat layered
intrusion in the west (e.g., Alapieti et al., 1990). The rocks of the Unijoki Formation display
volcanic textures such as a very fine grain size, amygdules, and shattered plagioclase
phenocrysts. The Sirniövaara Formation displays similar textures and is also considered to be
volcanic in origin, although the volcanic traits are not as clear as in the Unijoki Formation.
The absence of typical textures of intrusive granophyres (e.g., Hughes, 1971; Smith, 1974)
also argues for the volcanic origin of the Sirniövaara rhyodacite (II).
7.3. Early Paleoproterozoic volcanism in the northern Fennoscandian shield
In addition to the emplacement of the mafic layered intrusions, the early Paleoproterozoic
extension in the Fennoscandian shield was manifested by voluminous volcanism that ranged
28
from komatiitic to rhyolitic (e.g., Manninen, 1991; Puchtel et al., 1997; Lehtonen et al., 1998;
II). In northern Finland, the ~2.44 Ga volcanic rocks are combined into the Salla Group
(Lehtonen et al., 1998), in Russia they are commonly referred to as the Sumi Group in Karelia
or the Strelna Group in the Kola area (e.g., Zagorodny et al., 1982; Gorbunov et al., 1985).
The rocks of the Salla Group are found throughout northern Finland, commonly as the
lowermost volcanic formation lying unconformably on the Archean basement, or forming
the roofs of some of the 2.44 Ga layered intrusions (Mutanen, 1997; Lehtonen et al., 1998).
The U–Pb zircon ages indicate that the Salla Group is 2.48 to 2.44 Ga old (Manninen et al.,
2001; Mutanen and Huhma, 2001; Räsänen and Huhma, 2001). Age determinations available
for the Sumian volcanic rocks in the Paanajärvi area near the Oulanka layered complex in
Russia show that these rocks are also ~2.44 Ga old (Buyko et al., 1995). It has been suggested
that some of the Paleoproterozoic volcanic rocks (e.g., the Vetreny belt, Russia; Puchtel et al.,
1997) could represent extruded equivalents of the mafic layered intrusions, but the question
remains open.
The Sirniö Group in Koillismaa is an example of Paleoproterozoic volcanism intimately
associated with a layered intrusion and is tentatively correlated to the Salla Group in northern
Finland (II). The Sirniö Group is interpreted to be slightly older than the Koillismaa layered
igneous complex and thus to represent the original roof of the mafic intrusion (II). The
hydrothermal alteration of the Sirniövaara rhyodacite and the formation of the granophyric
intergrowth texture are thought to result from the heating and fluid flow associated with the
emplacement of the gabbroic rocks beneath the volcanic pile. Trace element data and Nd
isotope composition of the rocks of the Sirniö Group suggest that these rocks could represent
melts formed from the mafic magmas in the lower crust, possibly by an AFC process (III).
The absence of inherited Archean zircons also indicates a more juvenile source for these
rocks. The rift-related magmatism was probably initiated with the volcanism, as the relatively
light silicic melts were able to rise through the crust via zones of extension.
7.4. Early Paleoproterozoic silicic plutonism in the northern Fennoscandian shield
The data on silicic plutonism associated with the ~2.44 Ga rifting of the Fennoscandian
shield is relatively sparse. In the Karelian Province, the three known occurrences of silicic
plutonic rocks of this age are the 2435±12 Ma Kuhmo granites in eastern Finland (Luukkonen,
1988), the 2450±72 Ma Nuorunen granite in Oulanka area, northwestern Russia (Hackman
and Wilkman, 1929; Buyko et al., 1995), and the 2442±3 Ma Kynsijärvi quartz alkali feldspar
syenite in Koillismaa (I). These intrusions are situated ~100 km apart from each other (Fig.
1). Some small occurrences of A-type granites of similar age are also known in the Belomorian
belt (Lobach-Zhuchenko et al., 1998). The Kynsijärvi quartz alkali felsdpar syenite and the
Nuorunen granite are both intimately associated with mafic layered intrusions, the Koillismaa
and Oulanka complexes, respectively, and the Kuhmo granites are thought to be connected to
a diabase dike swarm, also indicating an extensional emplacement environment for these
granites (Luukkonen, 1988). All these ~2.44 Ga silicic intrusions show A-type geochemical
characteristics such as high alkali abundances, high FeO*/MgO, Zr, Nb, Zn, Ga, REE and a
negative Eu anomaly. Other geochemical traits indicate unique sources for each of the three
intrusions. The Kynsijärvi quartz alkali feldspar syenite shows considerably lower SiO2
contents of ~71 wt.% and higher alkali abundances compared to the high-silica (>75 wt.%),
metaluminous Nuorunen granite and the peraluminous Kuhmo granites (I). Nd isotope data
for the Kynsijärvi and Nuorunen plutons also indicate that these rocks had different sources
(Fig. 11; III).
A-type magmas are thought to originate either from the partial melting of the lower crust,
mainly amphibolitic or granulitic, in some cases ferrodioritic rocks (e.g., Collins et al., 1982;
29
Rudnick and Fountain, 1995; Frost and Frost, 1997) or to be the products of extreme
fractionation of mantle-derived mafic melts (e.g., Collerson, 1982; Foland and Allen, 1991;
Turner et al., 1992; Smith et al., 1999). The petrological modeling on the Kynsijärvi quartz
alkali feldspar syenite indicates that the most probable alternative in this case is the AFC
process involving the mafic mantle-derived magma and a lower crustal assimilant (III). An
A-type geochemical character is often associated with an anorogenic and extensional tectonic
setting (e.g., Eby, 1990). In the northern Fennoscandian shield, the A-type silicic plutonic
rocks, the mafic layered intrusions and the rift-related volcanic rocks, which also show Atype geochemical characteristics, form a typical magmatic association marking the rupture of
the Archean craton in the early Paleoproterozoic.
8. Conclusions
This work leads to the following conclusions:
1. The Archean nucleus of Fennoscandian shield was rifted during the early Paleoproterozoic,
the rifting was caused by mantle upwelling beneath the craton.
2. During the rifting, an anorogenic magmatic association consisting of mafic layered
intrusions, silicic plutons, and rift-related volcanic rocks was emplaced throughout the northern
Fennoscandian Archean craton. In the Koillismaa area, these rocks comprise the 2436±5 Ma
mafic Koillismaa layered igneous complex, the 2442±3 Ma Kynsijärvi quartz alkali feldspar
syenite, and the volcanic Sirniö Group. In the adjoining Oulanka area in Russia, a similar
magmatic association of the Oulanka complex layered intrusions, the Nuorunen granite, and
the Sumian volcanic rocks is found.
3. The age of the Archean complex that hosts the early Paleoproterozoic rocks in Koillismaa
is between 2800 and 2700 Ma. The youngest Archean granites were emplaced in the gneissic
basement at ~2.69 Ga, which also coincides with the peak granulite facies metamorphism of
the gneisses.
4. The early Paleoproterozoic silicic intrusions and the felsic volcanic rocks show A-type
geochemical characteristics such as high total amount of alkalies, high Fe/Mg, Ga/Al, Zr, and
REE, and a negative Eu anomaly.
5. The rocks of the Sirniö Group suffered metamorphism and hydrothermal alteration that is
interpreted to have been caused by the emplacement of the Koillismaa layered igneous complex
under the Sirniövaara Formation.
6. The Nd isotope composition of the Kynsijärvi quartz alkali feldspar syenite [εNd (at 2440
Ma) of –4.8 and TDM age of ~3000 Ma] falls close to the evolution path of the Archean rocks
in Koillismaa [εNd (at 2440 Ma) between –5 and –8.5 and TDM ages between 2900 and 3100
Ma]. In the Sirniö Group, the εNd (at 2440 Ma) values between –1.9 and –2.5, and TDM ages
between 2800 and 2950, are close to the values acquired from the layered mafic intrusions.
The Nuorunen granite shows εNd (at 2440 Ma) values of –1.8 and TDM age of 2750 Ma, which
may reflect the Nd isotope composition of the local Archean crust.
7. The early Paleoproterozoic silicic rocks in Koillismaa may all have been derived from
mafic mantle-derived magmas by AFC processes involving lower crustal contaminants.
Acknowledgments
This study was initiated in 1998 when I started my M.Sc. work in the “Layered igneous
complexes in northern Finland” projent of the GTK under the supervision of Dr. Markku
Iljina. Although my Ph.D. work has been done at the Department of Geology, University of
Helsinki with the funding of the Academy of Finland (project #47690) and the University of
Helsinki, the fruitful collaboration with the GTK has continued over the years. Most of the
30
material used in this study has been provided by the GTK and many people have contributed
to this work. The thin sections and geochemical analyses were made at the GTK. Lassi
Pakkanen and Bo Johanson provided the mineral chemical analyses and most of the thin
section pictures. The whole staff of the Rovaniemi office of the GTK is acknowledged and
special thanks go to Jorkki Räsänen, a competent field geologist and a reliable co-author. As
for the other co-authors, I have been very lucky to have had a chance to work with people
such as Dr. Hannu Huhma, Dr. Irmeli Mänttäri, Tuomo Karinen, and Prof. O. Tapani Rämö.
Hannu, Irmeli, and the other people (Tuula Hokkanen, Marita Niemelä, and Arto Pulkkinen)
at the isotope laboratory of the GTK always welcomed me warmly to their lab and were
incredibly patient with a field geologist who was more used to wielding a hammer than doing
precision work with the laboratory equipment. During the years, numerous discussions and
days spent doing fieldwork with Tuomo have improved my knowledge on the geology of the
layered intrusions in general and of the Koillismaa area in particular. Tapani has been a great
supervisor who, despite his busy schedule, has always been ready to help and encourage. I
also want to acknowledge my other supervisor, Prof. Emeritus Ilmari Haapala, who provided
the original funding for the project. The people at the Department of Geology, University of
Helsinki, have provided a good working environment and refreshing coffee break
conversations. Kirsi-Marja Äyräs is thanked for keeping the administrational things of my
project in law and order. Dr. Paula Kosunen and Dr. Arto Luttinen have helped with many,
sometimes absurd-sounding, scientific problems that came up during the study. This work
was thoroughly reviewed and greatly improved by Dr. Matti Vaasjoki (GTK) and Prof. Eero
Hanski (University of Oulu).
Beside the academic community, I want to acknowledge the help and support of my family,
both alive and dead. The three people I miss the most are my father Hannu, my grandfather
L.O.Kuitunen, who, despite his commercial background, was very interested in natural
sciences, and my aunt Annukka, who taught me to appreciate both books and nature. But I am
especially indebted to my mother Nora Landen, my mother-in-law Sirkka Lauri and my dear
husband Hannu, who have taken care of the important things - such as our little daughter
Silja - while I have been busy with geology. Thank you all!
Espoo, 26.4.2004
Laura S. Lauri
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Appendix 1. Mineral compositions and mineral/melt partition coefficients used for
petrological modeling.
Mineral compositions used in Stage 1 of fractional crystallization
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
Total
Olivine
Sample 6Nä
Clinopyroxene
Sample 6Nä
38.96
13.61
0.18
47.18
0.07
100
54.25
0.15
2.07
4.36
0.16
17.84
20.65
0.52
100
Plagioclase
Bushveld bytownitea
49.18
32.22
0.24
0.20
15.42
2.57
0.17
100
Magnetite
Sample 16Po
9.93
0.21
89.66
0.14
0.06
100
Mineral compositions used in Stage 2 of fractional crystallization
Clinopyroxene
Sample 14Po
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
Total
Orthopyroxene
average of 18 analyses
51.71
0.53
1.69
11.01
0.26
14.16
20.64
100
54.41
0.22
1.21
16.09
0.28
25.69
2.09
0.02
100
Plagioclase
labradoriteb
Magnetite
Sample 16Po
52.63
0.09
29.82
0.45
0.08
12.70
4.02
0.21
100
9.93
0.21
89.66
0.14
0.06
100
Ilmenite
average of 13 analyses
0.07
47.85
0.07
50.19
1.52
0.25
0.25
100
Mineral analyses from the Koillismaa layered igneous complex (Alapieti, 1982), excepting the plagioclase:
Kracek, F.C., Neuvonen, K.J., 1952. Amer. J. Sci., Bowen vol., 93–318.
Gutman, J.T., Martin, R.F., 1976. Schweitz. Min. Petr. Mitt. 56, 55–64.
Analyses recalculated to 100%. FeO* denotes total Fe2+, - not detected.
a
b
Rhyolitic and dacitic Kd’s
Element Hbl
Bt
Pl
Afs
Cpx
Opx
Qtz
Ox
Zrc
Apa
All
Ti
Ilm
Nb
Zr
La
Nd
Eu
Yb
6.367
2.00
3.18
0.044
0.145
0.179
0.06
0.10
0.38
0.17
2.11
0.077
0.10
0.072
0.025
1.13
0.012
0.80
0.60
0.015
0.166
0.411
0.64
0.80
0.20
0.78
0.22
0.17
0.86
0.015
0.016
0.056
0.017
2.50
0.80
0.89
1.65
0.47
1.10
4500
2.00
2.20
3.14
270
0.1
0.10
20.0
57.1
30.4
23.1
2.00
2600
1620
110
30
6.3
4.00
-
1.00
7.10
7.60
2.50
4.10
4.00
1.40
0.90
2.89
3.44
4.89
Basaltic Kd’s
Element
Cpx
Pl
Ol
Ox
Nb
Zr
La
Nd
Eu
Yb
0.10
0.10
0.56
0.23
0.474
0.542
0.01
0.048
0.148
0.055
1.125
0.023
0.01
0.012
0.067
0.007
0.007
0.049
1.00
0.10
0.015
0.026
0.025
0.018
Kd’s: see references in Rämö (1991), Rollinson (1993) and Elliott (2003).
36