GEOLOGISKA FORSKNINGSCENTRALEN G E O L O G I C A L S U R V E Y O F F I N L A N D

Pentti Hölttä

High-grade metamorphic rocks at the bottom and on the

surface: granulites and evolution of the Earth's crust in

the Archaean of eastern Finland

GTK Espoo 2000

High-grade metamorphic rocks at the bottom and on the surface:

granulites and evolution of the Earth's crust in the Archaean of

eastern Finland

Pentti Hölttä

Geological Survey of Finland

Academic dissertation

To be presented, with the permission of the Faculty of Mathematics and Natural Sciences of the

University of Turku, for public criticism in the Tauno Nurmela Auditorium, Turku University Main

Building, ~unel7&, 2000 at 12 noon

High-grade metamorphic rocks at the bottom and on the surface: granulites and evolution of the Earth's crust in the Archaean of eastern Finland

Pentti Höltta

Abstract

Rocks that were metamorphosed in medium pressure at mid-crustal or lower crustal conditions are exposed as fault-bounded blocks in the Archaean Varpaisjärvi area, central Finland. In Kaavi, ca. 100 km SE from Varpaisjärvi early Palaeozoic kimberlite magmas have aiso brought to the Earth's c m t l xenoliths fkom mid or lower crust that were crystallized at almost similar PT conditions than the Varpaisjarvi granulites.

The Varpaisjärvi granulite area represents at least two accreted blocks or terranes that exhibit marked lithological, geochemical and age differences. The main differences are two contrasting groups of tholeiitic mafic granulites, the Jonsa block having only the Group II metatholeiites, and the presence of quartz-cordierite and cordierite-orthoamphibole/orthopyroxene rocks only in the Jonsa block which also has younger zircon U-Pb (2.63 Ga) and Sm-Nd TDM model ages (2.9-2.7 Ga) than other granulites (3.2-3.1 Ga). In all blocks the Sm-Nd garnet-whole rock ages are younger, ranging fiom 2.48-2.59 Ga, which evidently is the age of closure of the Sm-Nd system m these rocks. Partial melting of compositionally basaltic and andesitic rocks produced garnet, plagioclase and py- roxene bearing restitic granulites at 9-11 kbar and 800-900°C. 'Peak' conditions were followed by cooling and decompression to around 700°C and 7 kbar. These geochemical and age differences be- tween the blocks, as well as the clockwise PT path indicate that the Varpaisjärvi granulites forrned in the collision of exotic terranes. Granulites underwent a second metamorphic event at lower pressure conditions, which seems to be connected with the semibrittle fractwing of the bedrock either during the emplacement of the Palaeoproterozoic dolerites at 2.3-2.1 Ga or during the Svecofennian orogeny at 1.89-1.88 Ga.

The Lahtojoki kimberlite pipe in Kaavi, eastem Finland contains lower crustal mafic granulite - xenoliths which were crystallized at ca. 800-900'C and 7.5 - 12.5 kbar, which corresponds with mid- lower crustal depths of ca. 22-38 km. Chemical composition of the xenoliths suggest that their protoliths crystallized fiom basaltic melts. The U-Pb ion probe dating of zircons fkom two samples yielded variable ages for zircons even in the same xenolith, between ca. 2.6-1.7 Ga. These ages correspond with main late Archaean and Palaeoproterozoic orogenic events in the Fennoscandian shield. The Sm-Nd garnet-clinopyroxene isochron age fiom one dated sample is 1.6 Ga, which represents either a cooling or reheating of the lower crust by rapakivi magmatism. Because the petrology, geochemistry and ages of xenoliths do not correlate well with the nearest exposed Archaean mafic granulites, it is evident that the present lower crust, underlying the Archaean rocks, has a considerable Palaeoproterozoic component.

Key words (GeoRef, AGI): granulite, metamorphism, lower crust, PT path, geochronology, geochemis- try, Archaean

Pentti Höltta, Geological Survey of Finland, FIN-02150 Espoo, email peniii. holtta @gs$j?

ISBN 951-690-770-9

CONTENTS

Page

PREFACE 4

INTRODUCTION 5

DEHNlTION OF THE LOWER CRUST 6

COMPOSITION OF THE LOWER CRUST 7

LOWER CRUSTAL GRANULRE TERRAINS 7

Terminology 7

PT paths and tectonic settings of granulite terrains 8

GRANULITE XENOLITHS 9

COMPARISON OF THE VARPAISJÄRVI GRANULITES AND XENOLITHS IN LAHTOJOKI, KAAVI 11

A TECTONIC MODEL FOR THE EVOLUTION OF THE vARPAISJÄRVI AREA-13

CONCLUSIONS 14

ACKNOWLEDGEMENTS 15

REFERENCES 15

4

Preface

This paper includes the foilowing articles referred to with Roman numerals:

L Hölttä, P., 1997. Geochemical characteristics of granulite facies rocks in the Archaean Varpais- järvi area, central Fennoscandian Shield. Lithos 40, 3 1-53.

II. Hölttä P. and Paavola, J., 2000. P-Tt development of Archaean granulites in Varpaisjärvi, Cen- tral Finland, 1: Effects of multiple metamorphism on the reaction history of m&c rocks. Lithos, 50, 97- 120.

IIL Hölttä, P., Huhma, H., Mänttäri, 1. and Paavola, J., 2000. P-T-t development of Archaean granulites in Varpaisjärvi, Central Finland, II: Dating of high-grade metamorphism with the U-Pb and Sm-Nd methods. Lithos, 50, 121-136.

IV. Hölttä, P., Huhma, H., Mänttari, I., Peltonen, P. and Juhanoja, J., 2000. Petrology and geo- chemistry of mafic granulite xenoliths from the Lahtojoki kimberlite pipe, eastem Finland. Lithos, 51, 109-133.

Paper 1 deals with the geochemistry of the Varpaisjärvi granulites, which are evidently the best

examples of exposed lower crustal rocks in Finland. The Varpaisjärvi area represents fault-bounded

blocks or terranes that exhibit marked lithological and geochemical differences. The main

differences are the trace and rare earth elment compositions of tholeiitic mafic granulites which

vary, as some lithologies fkom one terrane to another. This is interpreted as a result of Archaean

terrane accretion.

Paper II focuses on the metamorphic evolution of the Varpaisjärvi mafic granulites. They were

metamorphosed at around 800-850°C and 9 kbars, the northwestem part of the granulite area shows

even higher pressures. Metamorphic reactions indicate a PT path where cooling took place after

melting simultaneously with decompression. This kind of clockwise PT path is typical for colli-

sional orogenies. The Varpaisjärvi granulites underwent a second metamorphic event during the

Proterozoic, which makes the timing of some metamorphic reactions problematic.

Paper III focuses on the timhg of metamorphism with the U-Pb zircon and monazite and the Sm-

Nd garnet-whole rock methods. The conventional zircon datings show that the Varpaisjärvi granu-

lites were metamorphosed at ca. 2.63 Ga. The Sm-Nd ages are a l l younger than 2.63 Ga and record

cooling. Granulite metamorphism was not able to reset 3.1-3.2 Ga zircons in the northern blocks.

The different zircon U-Pb ages, as weil as different Sm-Nd model ages in various blocks support

the idea that the granulite area was formed as a result of accretion of exotic terranes of different age.

Paper IV is a study of lower crustal xenoliths that were brought to the Earth's surface by kimber-

lite rnagma in Kaavi, ca. 100 km SE from the Varpaisjärvi granulites. These xenoliths were crystal-

lized at ca. 800-900°C and 7.5 - 12.5 kbar, which corresponds with crustal depths of ca. 22-38 km.

Chemical composition of the xenoliths suggest that their protoliths crystallized fiom basaltic melts,

with some K-enrichrnent. The U-Pb ion probe dating of zircons from two samples yielded variable

ages for zircons even in the same xenolith, between ca. 2.6-1.7 Ga. These ages correspond with

main late Archaean and Palaeoproterozoic orogenic events in the Fennoscandian shield.

These papers have done in co-operation with the co-authors. In all papers P. Hölttä has compiled the

article and is responsible for the geological descriptions, interpretations and discussions. In Paper

II and Paper 111 J. Paavola has made the metamorphic map. In Paper III and Paper IV H. Huhma

and 1. Mänttäri are responsible for the isotope analyses and partly for the mterpretation of the data.

1x1 Paper IV P. Peltonen and 1. Mänttäri are responsible for the zircon analyses and partly for the

interpretation of the geochemical &ta. J. Juhanoja is responsible for the cathodoluminesence imag-

ing in Paper IV.

The lower crust has been a continuous subject of interest for earth scientists because of its impor-

tance in continental crustal evolution. It is possible to measure the physical properties of the lower

crust indjrectly using various geophysical techniques, but for a petrologist the lower crustal rocks

are available fiom two main sources: granulite terrains and xenoliths which have been carried to the

Earth's surface by rapidly erupting alkaline and basaltic magmas.

In Finland there are a few exposed terrains with granulites that could have been metamorphosed in

the lower crust. In the Palaeoproterozoic Lapland granulite belt the metamorphic pressures at the

thermal maximum were 6-12 kbar (Raith & Raase, 1986; Belyaev & Kozlov, 1997) and in the Ar-

chaean Varpaisjärvi granulite complex the 'peak' metamorphism took place at around Wl kbar

(Paavola, 1984; Paper II). These pressures suggest metamorphic crystallization at a depth of ca. 20-

35 km. In the Lapland granulite area the thickness of the modern c m t is 42-46 km (Korja et aL,

1993); therefore at least the highest pressure Lapland granulites were metamorphosed in the lower

crust.

The Varpais~arvi area is the best example in Finland of an Archaean exposed granulite terrain, con-

sisting of various lithologies and a having a complex evolution fiom mid-Archaean to Palaeopro-

terozoic (Paper 1; Paper II; Paper III). In the area where the Varpaisjärvi granulites crop out the

crust is thick, close to 60 km, in which case the 9 kbar granulites, metamorphosed at the depth of

ca. 30 km would represent only mid-crustal conditions. However, the thickening of the crust was

evidently a Proterozoic event (Korja et aL, 1993; Paper IV), therefore the Varpaisjärvi granulites

can represent lower crust during the Archaean, especially because we do not know any Archaean

rocks in the Karelian craton which were crystallized at considerably higher pressures. In the Ar-

chaean of eastern Finland there also exist a few smaller localities (Lieksa, Murhijärvi) where granu-

lite facies rocks occur, but the PT data fiom those areas are insdcient this far.

During the 1s t decade, diamond prospecting has revealed several kimberlites in eastern Fiuland. A

kimberlite cluster is located in Kaavi, around 100 km SE of Varpaisjärvi, intruding through the Ar-

chaean upper crust and overlying Palaeoproterozoic sediments. These kimberlites contain both man-

tle and crustal xenoliths. Garnet-clinopyroxene Sm-Nd ages of garnet-bearing mantle xenoliths

range fiom 525-607 Ma. The minimum age is believed to approximate the age of kimberlite mag-

matism which would be early Palaeozoic, mid-late Cambrh (Kukkonen & Peltonen, 1999; Pelto-

nen et al., 1999). The crustal xenoliths yield pressures of 7.5-12.5 kbar. Because the crust in the

Kaavi area is exceptionally thick, exceeding 60 km (Korja et aL, 1993), these pressures indicate that

the xenoliths originate fiom mid crust and upper parts of the present lower crust, fiom depths of ca.

23-38 km (Paper IV), following the definition of the lower crust by Korja et aL (1993).

The pwpose of this paper is to discuss the lower crustal evolution of the Archaean area of eastern

Finland in the light of these two examples, the Varpa i s j i granulite terrain and the granulite xeno-

liths in the Lahtojoki kimberlite in Kaavi

Definition of the lower crust

What is the lower crust? Rudnick (1992) dehed the lower crust simply as the lower half of the

crust. According to Christensen & Mooney (1995), based on seismic velocity, the lower crust is a

region where v, varies between 6.8 and 7.8 kmh. Holbrook et al. (1992) gave a more detailed defi-

nition:

(1) In regions where there is no sharp velocity discontinuity (Conrad) near the middle of the crust,

the lower 50% of the crust represents the lower crust.

(2) In regions where there is one velocity layer beneath the Conrad discontinuity, that layer is de-

fined as the lower crust.

(3) If there are two distinct velocity layers beneath the Conrad discontinuity, the middle crust is de-

fined as the layer immediately beneath the Conrad, the lower crust as the layer immediately

above the Moho.

(4) Where the velocity stmcture suggests a natural division of the crust into thirds, the lower crust is

defined as deepest third, the middle crust as the middle third.

According to Korja et al. (1993) the upper interface of the lower crust m the Fennoscandian shield

is the depth at which the seismic P-wave velocity exceeds 7 W s . This depth varies, but for example

at the seismic Baltic Profile which is close to the areas of this study it is between 30-35 km.

Composition of the lower crust

Precambrian shields and platforms have generally a thick crust (43 km) with a high-velocity layer in

the lower third of the crust. The lower crust is thought to consist of granulite facies rocks and to

have a mafic bulk composition, although there are significant regional variations from one area to

another. Restitic granulite facies metapelites, which have lost granitic melt fraction, have also high

seismic velocities and cannot be distinguished seismically from mafic granulites. However, pre-

dominance of high-velocity minerals (garnet, pyroxene, olivine) in the lower crust is necessary to

explain the increasing P-vawe velocities. In many regions seismic reflections are observed in the

lower crust, which may be due to compositional variation, although this feature has also been ex-

plained as a result of seismic anisotropy or existence of pore fluids. However, the presence of in-

termediate and felsic rocks in most xenolith suites supports the existence of these lithologies in the

lower crust. In areas where sections of the crust are exposed, from upper to lower crust (e.g. the

Ivrea zone and Calabria in Italy), the metamorphic grade increases fiom greenschist facies in the

upper section to granulite facies in the lower crustal section. In the upper and middle parts of these

sections felsic and intermediate lithologies dominate, and the lower parts consist of magmatic rocks

(gabbros, anorthosites, norites, pyroxenites etc.) and granulite facies mafic and metasedimentary

gneisses. Mafic granulites form a dominant part of lower crustal xenoliths (Fountain & Christensen,

1989; Holbrook et aL, 1992; Percival et aL, 1992; Downes, 1993; Rudnick & Fountain, 1995). Ac-

cording to Rudnick & Fountain (1995) the lower crust approaches the composition of a primitive

basalt, having a high Mg# (100Mg/Mg+Fe), high Ni and Cr contents, and low abundances of heat-

producing elements. The lower crust is light rare earth element enriched.

Lower cnistal granulite terrains

The word 'terrain' is used in this paper as a purely geographical term. The popular term 'terrane' was

defined in the Glossary of Geology (Bates & Jackson, 1980), meaning a group of rocks and an area

where they crop out. Later the terms terrane and terrane analysis have acquired more specific defmi-

tions, sometirnes giving rise to controversy and confusion. According to Jones et al. (1983) and

Jones (1990) the term terrane refers to fault-bounded geologic entities of regional extent that daer

in significant ways fkom their neighbours. Such differences between terranes imply relative tectonic

dislocations sufficiently large that sedimentary facies connections are broken and original palaeo-

geographic relations obscured. On the other hand, Sengör & Dewey (1990) criticised the usage of

the word terrane as a lump term for a number of older and more informative non-genetic and ge-

netic terms. They argued that the genetic terms nappe, extensional allochthon, strike-slip sliver,

fkagrnent, and the non-genetic but geometrically informative terms sliver and block are preferable to

terrane.

The term granulite is used as a general term for granulite facies rocks that were metamorphosed at

ca. 700°C or higher temperatures. Not all granulites represent the lower crust. Low pressure granu-

lites, metamorphosed at pressures lower than ca. 6 kbars can represent mid-crustal conditions where

the high temperature was caused by elevated heat flow due to upwelling of the asthenosphere in re-

sponse to lithospheric thinning or by advective heat flow fkom magmas. In central and southern

Finland the Palaeoproterozoic Svecofennian granulites represent these low-pressure high-

temperature domains.

PTpaths and tectonic settings of granulite terrains

Granulite terrains may cover hundreds or thousands of Square kilometres (e.g. southern India,

Napier Complex, Antarctica), or they may occur as slices, slabs or fiagments like the Calabrian

rnassif or Ivrea zone in Italy (Rudnick & Fountain, 1995; Harley, 1989; and references therein).

Harley (1989) divided granulites into two major groups on the basis of the PT paths that they have

recorded. Mineral reactions, textural relationships and geothen11obarometry indicate that many

granulites have undergone near-isothermal decompression after their apparent thermal maximum

whilst other terrains show evidence for near-isobaric cooling. There are also terrains which have

undergone a more complex PTt path as a consequence of overprinting by subsequent metamorphic

events.

The PT paths of granulite terrains seem to be correlated with their lithological compositions. Ter-

rains that were isotherrnally uplifted are dominated by felsic compositions, whereas those that have

cooled isobarically have a significantly larger component of maiic lithologies (Rudnick & Fountain,

1995). Isothermally decompressed granulite terrains may represent crust double-thickened due to

collision and exhumed by erosion andlor extensional collapse of the resulting orogen. On the other

hand, granulites that show evidence for isobaric cooling are often regarded as products of mag-

matic underplating. Uplift of granulites formed in this way may be unrelated to the event that

caused the granulite facies metarnorphism. Geochronological constraints, when available, support

long-term residence of isobarically-cooled terrains in the deep crust (Wells, 1980; Bohlen &

Mezger, 1989; Bohlen, 1991; Mezger, 1992; Rudnick & Fountain, 1995). Consequently, these

probably provide better examples of lower crust than the isothermally decompressed granulites in

which the high temperatures and pressures that they record are only a transient feature (Rudnick,

1992; Rudnick & Fountain, 1995).

Accordmg to Percival et aL (1992), there are four different mechanisms that can transport lower

crustal sections to the Earth's surface. These are compressional, extensional (e.g. core complexes),

transpressional and impactogenic uplifts. There are also wide oblique transitions from &reenschist to

granulite facies where the uplift mechanism is problematic. Several features are common to all

granulites interpreted as sections of the lower crust, regardless of age, tectonic setting or uplift his-

tory. Metamorphic pressures at the base of the section are 7-9 kbar although values as high as 12-14

kbar have been reported. Magmatic rocks form a dominant part of lithologies. Widespread migma-

tites indicate partial melting at the prevaihg metamorphic temperatures (700-850°C iuferred from

thermometry). According to Percival et al. (1992), tectonic setting that is plausible for most exposed

lower crustal sections is a magmatic arc environment, superimposed on a pre-existing basement or

supracrustal sequence. Because most granulite terrains have a present 30-40 km crustal thickness, m

contrast to 4 0 km values for mature island arcs, a continental Andean-type arc environment is

probable during the late magmatic phase of crustal development.

Granulite xenoliths

Downes (1993) has reviewed the main petrological and geochemical features of lower crustal xeno-

liths in Europe. The most cornmon lithologies are m&c granulites composed of clinopyroxene, or-

thopyroxene and plagioclase. Many of these xenoliths are formed as cumulates from underplated

basaltic magmas (Rudnick, 1992). Within these mafic metaigneous suites there are differences, e.g.

subgroups which are either rich in MgO representing pyroxene-rich cumulates, or rich in A1203 rep-

resenting plagioclase-rich cumulates (Kempton & Harmon, 1992). 1x1 many cases, isotopic data in-

dicate interaction between mantle-derived melts and those formed fiom pre-existing crust (Downes,

1993). In most xenolith suites there are some examples of granulites with intermediate compositions

(Si02 between 55 and 60 wt.%), both from felsic igneous rocks and metasediments. Most lower

crustal xenoliths, regardless of mineralogy, are LREE-enriched. Many show positive Eu-anomalies

and are probably plagioclase-rich cumulates. Metasedimentary xenoliths have less variable REE

pattems than metaigneous ones because sedirnentary processess average the REE pattems of eroded

rocks (Downes, 1993). Geobarometry gives crystallization pressures of 6-9 kbars; for garnet-

bearing mafic granulites pressures are higher, exceeding 10 kbars. In the thick crust of the Fenno-

scandian Shield the xenoliths, which in the review of Downes (1993) are all from the White Sea re-

gion of the Kola Peninsula, are mostly eclogites or retrogressed eclogites, whereas those fiom thin-

ner Phanerozoic crust are alrnost all granulites. Garnet and clinopyroxene are the main constituents

both in garnet pyroxenites and eclogites, the main difference between these two rock types being

the Na content of clinopyroxene (Na-rich in eclogites and Na-poor in garnet pyroxenites and in gar-

net granulites).

The geochronological evidence for the protolith age of the lower crust, based on dating of the xeno-

liths is not well constrained. The zircon datings are in practice possible only using the single-zircon

techniques, because other age determhations methods are likely to date cooling or late thermal dis-

turbances. However, in their study from north Queensland, Australia, Rudnick & Williams (1987)

noticed that the U-Pb zircon ages of xenoliths correlate well with the surface geological history, and

the same xenolith might have both Proterozoic and Palaeozoic zircons. Similar results were reported

by Chen et aL (1994) from xenoliths in South Australia.

Which are better representatives of the lower crust, xenoliths or exposed granulite terrains? Xeno-

liths may have been &rived from high-grade metamorphic rocks already located at higher crustal

levels than the pressures at which they equilibrated. Thus they may not necessarily represent the

contemporary lower crust at the time of the magmatic event. The rising magma may also have taken

xenoliths preferentially from one horizon and other horizons may have not been sampled at all

(Downes, 1993). This may explain the significant differences in the compositions of xenoliths and

granulite terrains. Xenoliths seem to be dorninated by mafic lithologies (Si02 in general c 55 wt.%)

whereas in many granulite terraius an evolved intermediate or felsic component dominates (Rud-

nick & Fountain, 1995). On the other hand, in exposed lower crustal sections the mafic igneous

component increases in the lower parts, the upper parts being more felsic (Percival, 1992). Strong

comparisons can also be drawn between some exposed cross sections of the lower crust such as the

Ivrea zone in NW Italy and granulite xenoliths from the Massif Central area in France (Downes,

1993). Increasing average seismic velocities with depth indicate increasing proportions of mafic

rocks. The average composition varies between different tectonic provinces, but the composition of

the bulk lower crust approaches that of a primitive basalt, although felsic and intermediate litholo-

gies can occur locally and cause seisrnic reflections observed in some localities (Rudnick & Foun-

tain, 1995). In that sense the xenoliths may give a better idea of the bulk composition of the lower

crust than most exposed granulite terrains.

Comparison of the Varpaisjärvi granulites and xenoliths in Lahtojoki, Kaavi

Paper IV presents data on granulite xenoliths fiom the Lahtojoki kimberlite in Kaavi, which is lo-

cated ca. 100 km SE from the Varpaisjärvi granulite area. In this region the crust is exceptionally

thick, close to 65 km (Korja et al, 1993). The highest pressures of the xenoliths, given by geo-

barometry, are around 12-13 kbars, which corresponds to depths of ca. 35-40 km. Assuming this

pressure represents the original crystallization depth and the crust was thickened during the Protero-

zoic, these xenoliths did not originate fiom the lowermost third of the crust where the seismic vp in

that area is 7.27-7.386 (Korsman et al., 1999).

The xenoliths have many prorninent diiTerences both in geochemistry and petrology compared with

the Varpaisjärvi granulites. With few exceptions, T i a , K2O and P20s contents are higher in the

xenoliths than in the Varpais~anri granulites. Most of the mafic granulites in Varpaisjärvi corre-

spond in composition with andesites and basalts, whereas the Lahtojoki xenoliths are subalkaline

basalts (Paper 1, Paper IV). The Ba contents are clearly higher in the xenoliths than in most mafic

granulites, although some of the Group II mafic granulites in the Jonsa block (Fig. 1) have as high

Ba as the xenoliths. The LREE enriched rare earth element patterns of the xenoliths differ clearly

from those of the Group 1 (older) mafic granulites in Varpais~ärvi, but most of them have a similar

trend with the REE patterns of the Group II (younger) rnafic granulites although the LREE have a

wider scatter in the Varpaisjärvi rocks.

Orthopyroxene is rare in the xenoliths whereas it is present in almost all mafic granulites in Var-

paisjärvi. The sodium contents of clinopyroxenes are higher in the xenoliths. Consequently, the

pressures given by geobarometry are higher in most xenoliths than in the Varpaisjärvi granulites,

indicating crystallization at deeper crustal levels.

Finally, the zircon ages show that the Kaavi xenoliths do not originate fiom the Varpaisjärvi-type

Archaean granulites. There are no Proterozoic zircons in the Varpaisjärvi mafic granulites, whereas

one of the analysed xenoliths had only Proterozoic zircons and another both Archaean and Protero-

zoic zircons. The Sm-Nd TDM model ages of the mafic granulites are 2.7-2.9 Ga in the Jonsa block

but ca. 3.2 Ga in the other granulites in Varpaisjärvi, but in the xenolith where it was possible to

Legend:

Archaean rocks

1. A generalised geological map of the Iisalmi block.

3 1-5 = Archaean rocks; 1 = granulite facies areas; 2 = maiic gianulites, in the Jonsa block with AI-Mg

Proterozoic rocks granulite interlayers; 3 = arnphibolites, cordierite-

orthoamphibole rocks, metasediments; 4 = granitoids; 5 = enderbites;

9 6-1 1 = Proterozoic rocks: 6 = granitoids; 7 = gabbros; 8 = quartzites and schists; 9 = serpentinites; 10-1 1 =

diabases; 12 = shear zone (mainly oblique-slip and strike-slip shear); 13 = shear sense; 14 = thrust fault. The heavy dotted line separates the lisalrni and Rauta- vaara terranes. Ksb in the inset = Kainuu schist belt.

calculate the model age was 2484 Ma, indicating Palaeoproterozoic origin. The Sm-Nd garnet-

whole rock ages are ca. 2.5-2.6 Ga in Varpaisjärvi and ca. 1.6 Ga in a xenolith, although this may

reflect only slow cooling of the xenolith materia1 in lower crustal conditions. However, the age data

of the xenoliths indicates that the lower crust under the Archaean was mainly formed during the

Svecofennian orogeny by underplating of mafic rnagmas and their mixing with older materiaL

A tectonic model for the evolution of the Varpaisjanii area

A generalised geological map of the central part of the Varpaisjärvi area, based on the mapping of

Paavola (1980, 1987, 1990, 1997) is presented in Fig. 1. On the basis of the geochemicai, isotopic

and metamorphic data, the tectonic evolution of this complex resembles that of the Superior and

Slave Provinces in Canada, which were formed as a result of accretion of fundamentally Merent

tectonostratigraphic terranes. Several fault-bounded subprovinces of Merent age and lithologies

can be found in the Superior Province, which was formed as a result of accretion of oceanic and

continental volcanic arcs, accretionary sedimentary wedges, older microcontinetal fragments etc. in

a convergent margin setting (Card, 1990; Percival et al., 1994). In the Slave Province, regions com-

posed of Merent rock suites are separated by high-strain zones recording large displacements;

these zones separate terranes that have been juxtaposed during collisional orogenesis (Kusky,

1989). Similar processes may have operated in the Varpais~arvi area (Papers 1-111).

The western part of the area, the Iisalmi terrane (or the Iisalmi complex as defined by Lundqvist et

al., 1996) is composed of 3.2 Ga mafic oceanic arc-type volcanic rocks, which were juxtaposed in a

collision at 2.7 Ga with the Rautavaara terrane which forms the eastern part of the area (Fig. 1).

This lead to crustal thickening and first partial melting, indicated by the 2.7 Ga zircon ages of leu-

cosomes in Kumisevanmäki (Höltta & Mänttäri, unpubiished ion probe data). The usage of the term

'terrane' following the definition of Jones (1990) is justified in this context, because in the Rauta-

vaara terrane the rocks are younger than in the Iisalmi terrane. The oldest Sm-Nd model ages in

Rautavaara are younger than 3.0 Ga and zircon ages of retrogressed granulites are around 2.6-2.65

Ga as in the Jonsa block (Höltta, Huhma & Mänttäri, unpublisehd data). Also lithologies and geo-

chemical features of mafic rocks in the Rautavaara terrane are dBerent from the Iisalmi terrane

comprising more sedimentary rocks, hydrothermally altered cordierite-orthoamphibole/orthopyro-

xene rocks and quartz-cordierite rocks which are lacking in the Iisalmi terrane.

The terrane boundary runs across the granulite area separating the Jonsa block from other granulites

(Fig. l), which means that the accretion was pre- or early metamorphic. Large areas east of the Var-

paisjärvi granulites have lithologies similar to those of the Jonsa block belonging evidently to the

same Rautavaara terrane. Those areas were strongly retrogressed in Proterozoic pervasive deforma-

tion and amphibolite facies metamorphism, which also makes the mapping of the actual terrane

boundary difficult. A comprehensive structural analysis is lacking fiom the Varpaisjärvi area, but

mainly NW-SE trending Archaean structures and higher pressures in the N W part of the area indi-

cate thrusting fiom the SE.

Thickening was followed by magmatic input of heat fiom enderbites at ca. 2.63 Ga, which in some

places overprints the earlier migmatisation. The conventional zircon age of the enderbites is 2.68

Ga, which made the relationship between granulite facies metamorphism and enderbitic magmatism

problematic (Paper II). The ion microprobe data reveal a heterogenous zircon population in the

dated enderbite, giving ages fiom ca. 2.75-2.63 Ga, in which case the youngest zircons could give

the age of the emplacement. This was followed by erosion andlor extension, leading to near-

isothermal decompression. Because the Sm-Nd mineral ages record a slow cooling (Paper EI) the

granulites were not uplifted close to the erosion surface between 2.6-2.5 Ga but stayed at mid-

crustal levels. Their exhumation to the present erosion level might have taken place in the Protero-

zoic events, which strongly overprint the Archaean metamorphism. The granulite blocks are bor-

dered by Proterozoic shear zones, but some of these must have been active already in the Archaean

because they separate Archaean terranes.

- the Varpaisjarvi area was formed as a result of accretion of terranes of different ages during the

late Archaean. Terrane accretion was followed by granulite facies metamorphism which took

place at around 2.63 Ga. The Archaean PT path of the granulites was clockwise, typical for col-

lisional orogens. The Varpaisjärvi area underwent another metamorphic event during the Pro-

terozoic which had a strong effect on the present metamorphic textures and mineral assem-

blages of the granulites.

- the Varpaisjani granulite area is dominated by mtermediate lithologies. The PT path and the

presence of supracrustal rocks indicate that these granulites are not good representatives of the

deep crust. They are more likely upper crustal rocks that have been through an orogenic cycle.

- the age, mineralogy, geochemistry and PT conditions of the Varpaisjärvi granulites differ from

those of the mid-lower crustal xenoliths in the Lahtojoki kimberlite in Kaavi These xenoliths

mostly originate fiom the present mid-crust and do not represent the lowermost third of the crust

either. Their ages and compositions indicate, however, that the lower half of the crust under the

Archaean craton had a substantial magmatic input during the Proterozoic.

Acknowledgements

The author wants to express his gratitude to his employer, the Geological Survey of Finland and the

Director General Raimo Matikainen for giving a chance for these Studies. Prof. Kalevi Korsman is

thanked especially for suggesting the research topic and for various kinds of help during the work.

Many thanks also for the CO-authors, Hannu Huhma, Irmeli Mänttäri, Jorma Paavola, Petri Peltonen

and Jyrki Juhanoja, and for people who have been involved m the analytical and field work, espe-

cially Jukka Eskelinen, Jukka Jokela, Lassi Pakkanen and the staff of the Bedrock and Mineral Re-

sources Group and the Laboratory of Chemistry of the Geological Survey. Stephen Daly, Annakaka

Korja, Heikki Papunen and Markku Väisänen are thanked for helpful comments for this manuscript.

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