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Geological Survey of , Bulletin 304

THE PREC'AMBRIAN IN FINLAND

BY

AHTI SIMONEN

with 35 figures and 3 tables in the text

GEOLOGINEN TUTKJMUSLAITOS ESPOO 1980 Simonen, Ahti: 1980 The in Finland. GeologicalSurvey of Finland, Bulletin 304. 58 pages,. 35 figures, 3 tables.

The Precambrian crust, of Finland forms the central part of the Baltic , which is the. most extensive Shield area in . The geological evolution of the has been outlined to show the structural setting of. the Finnish Precambrian. The main geological and structural outlines of the Finnish Precambrian crust are based on lithological, structural, geochronological, seismological, gravimetric and aerornagnetic data. The .petrographical, structural, strati­ graphical and geochronological data on the principal geological units are presented in chronological order from the oldest to the youngest complexes; and the concluding remarks summarize the geological evolution of the Pre­ cambrian in Finland. The most important events in the geological evolution of the Finnish Pre­ cambrian were the revolutions that produced the Presvecokarelidic and Sveco­ karelidic folded areas 2 800-2600 Ma and 1 900-1 800 Ma ago,respectively. Most of the Finnish Precambrian rocks received their present form during these ancient events. Both of the orogenic revolutions were followed by periods of cratonization connected with lively igneous activity taking place on a stable in the. time intervals 2 400-2 500 .NIa and 1 550-1 700 Ma ago, respectively. The oldest nonmetamorphic sedimentary cover of the Finnish Precambrian is represented by the (1 300-1 400 Ma).

Key words: treatise, evolution, igneous rocks, metamorphic rocks, stra­ tigraphy, geochronology, structure, Baltic Shield, Svecokarelidic , Precambrian, Finland

Prof.. Ahti Sissonen, Otakuja 3 D 46, 02150 Espoo 15, Finland

ISBN 951-690-111-.5 ISSN 0367-522x

Helsinki 1979. Valtion painatuskeskus CONTENTS

Introduction ...... 5 The Baltic Shield 5 The Precambrian in Finland 8

Geological and structural outlines 0 ••••••• 0 ••• 0 •••••• 0 •••••••••••• 0 • 9

Lithological data .... 0 •••• 0 •• 0 0 • 0 0 ••••• 0 • 0 ••••• 0 0 ••••••••••• 0 0 •••• 0 •• 0 • • • • • 9

Structural data 0 0 0 •• 0 0 •••• 0 0 0 ••••••••••• 0 •••••••••••• 0•••••••••••••• •. ••••••• 9

Geochronological data .. 0 • 0 0 •• 0 •••••• 0 ••••••••••• 0 •••••••••••• 00•••••••••• •• 12

Seismological data 0 •••••••••• 0 •••••• 00••••••••••••• •• 13

Gravimetric data .. 00 ••• 0 •• 0 •••••••• 0 o. o ••••••••••••••••••••• 0 •••••••••••• , 13

Aeromagnetic data ... o· •• 0 •••••••••••••••••••••••••••••••••••••••• 0 0 0 • 0 • • • • •• 14 Characteristics of the geological units 17

Presvecokarelidic 0 •• '.' •••••••••••••••••••••••••••••••••••••••• •• 17

Basement area in eastern Finland 0 •••••• 0 ••••• 0 ••• 0 ••• 0 0 0 •• 0 ••••••••••• 0 17

Basement area in northern Finland 0 •••••••••••••••••• 0 • • •• 19

Granulite arch 00 •••• o •• 0 0 0 ••• 0 •• 0 0 o ••• 0 0 •• o ••• 0 0 o. o •••• 0 •• 000••••••••••• •• 20

Presvecokarelian igneous rocks younger than the folded basement . 0•0•••••••••••• •• 21

Mafic igneous rocks 0 0 ••••••• 0 0 0 •• 0 0 0 0 •• 0 0 0 0 0 ••• 0 0 0 • 0 ••••• 0 0 0 •• 0 0 0 • 0 0 0 •••• 0 0 21

Carbonatite complex . 0 •••• 0 •••••••••••• 0 •••• 0 ••• 0 0 0 0 • 0 ••••• 0 •• 0 •••••• 0 • • • • •• 23

Svecokarelides . 0 • 0 0 ••••• 0 0 • 0 0 ••• 0 0 0 0 0 0 • 0 •••••••••••••••••••••••.••••••• 0 • • • • • • •• 23

I

Svecofennidic belt . 0 0· ••• 0 • 0 0 0 •••••• 0 ••••••••••••••••••••••• 0 •••• 0 0 • • • • •• 28

Plutonic rocks of the Svecokarelides . 0 .0 ••• 0 ••• o •••••• 0 .0 •••••••• o •••••• '. •••• •• 33

Postsvecokarelian igneous and sedimentary rocks . 0 •• 0 .0 ••• 0 • 0 •••• 0 •• 0 •• 0 ••••••• 0 •• 42

Rapakivi o. 0 0 • 0 •••• o ••••• o •• 0 ••••• 0 • 0 •••••••• 0 ••• 00 •• 0 •••••• 0 0 ••• o. 43

Mafic igneous rocks 0 ••••• 0 0 •• 0 0 ••••• 0 •••••••••••••••••••••••• ~•0••••••••• •• 45

Jotnian sediments o' •••••••• 000 ••• o ••• 00.00 •••• 0 •••••• 000 ••• 0. o. •• •• 47

End of the Precambrian 0 0 • 0 0 ••• 0 0 •••••• 0 0 • 0 0 •••••• 0 ••• 0 0 ••••••••• 0 • • •• 47

Concluding remarks on the evolution ... 0 • 0 ••••••••••••••••••••••••• 0 0 ••••••• 0 0 • • •• 48

Economic 0 •••••••• 0 • 0 ••• 0 ••••••••••••••••••••••••••• 0 ••••• 0 ••• 0 • 0 • • •• 52

Ore deposits 0 0 •• 0 •••••••••••••••••••••••••••••••••••••••••• 0 • • •• 54

Industrial and rocks ... 0 •• 00 o ••••• o •••••• 0 • 0 •••• 00 ••• 0 ••••• 0 .0 ••• 0 o. 56

Acknowledgments 0 • 0 .' 0 ••••• 0 0 • 0 0 •••• 0 ••••••••••••• 0 ••••••• 0 • 0 • 0 0 0 0 56·

Selected papers on the Precambrian in Finland . 0 •• 0 0 ••• 0 0 • 0 0 0 0 •• 00•000•00•000 •••• 0 •• 0 •• 0 57 »Prove all things; hold fast that which is good». 1 Thes 5.21

INTRODUCTION

This paper on thePrecambrian in Finland is tributed many provocative new ideas regarding based on the geological studies of many gener­ the evolution of the Precambrian in Finland. ations ,of geologists. The pioneers and great An abundance of new material has been regis­ masters of Precambrian geology, J. J. Seder­ tered and published. holm (1863-1934) and P. Eskola (1883-1964), The motto quoted. above served as a guiding blazed the trail for research on the oldest geo­ principle in the preparation of this paper, which logical history of the globe. New geological is intended to be a short synthetic review of the studies, geological re-mapping, and geophysical main characteristics of the Precambrian in Fin­ and geochronological data have increased our land. Furthermore, this paper will provide as knowledge about the Finnish Precarnbrian to a an explanatory text to a new geological map of fundamental extent. In particular, age deter­ the Finnish Precambrian (1: 1 000 000, under minations and many new studies on the structure preparation), and it is offered as a guide through and of certain key areas have con- the rocky labyrinth of the Finnish crust.

THE BALTIC SHIELD

The Precambrian crust in Finland is a Paleozoic and younger faults. In the east and typical and central part of the Baltic or Fenno­ south, the surface of the Baltic Shield dips scandian Shield, which is the broadest Shield gently under the sedimentary cover of the East­ area on the European . The Baltic European platform. The platform cover is Shield is an uplifted part of the wide basement composed of the Late Precambrian, Paleozoic area upon which the sediments of the North­ and younger sediments that have not undergone and East-European platform have been de­ . posited. The Precambrian basement of this so­ The Precambrian age of the Baltic Shield called Fennosarmatian platform is exposed only can be determined from the fact that the meta­ in the Baltic and Ukrainian Shields. morphic and plutonic complexes of the Shield The boundaries and main structu ral features underlie nonmetamorphic Late Precambrian of the Baltic Shield are represented in Fig. 1. (Jotnian and Eocambrian) and Cambrian sedi­ The northern and western parts of the Shield ments. The bedrock of the Baltic Shield rep­ are bordered by the Paleozoic Caledonidic resents a deeply denuded section of Pre­ , which in many places has cambrian mobile belts of different ages, and overthrust upon the Baltic Shield. The south­ studying it has proved a fruitful exercise toward western margin of the Shield is characterized by the development of ideas needed to understand 6 Geological Survey of Finland, Bulletin 304

~8 "1~2 I!!iiI 3 ~10 c::::=4 EZJ 11 ~5 12 ~6 13

o~ 100 200_...... I300 km

30° \ Fig. 1. Main structural units of the Baltic Shield. 1, Paleozoic and younger sedimentary rocks; 2, Paleozoic igneous rocks; 3, Caledonides. Precambrian unmetamorphic sediments: 4, Jotnian formations. Dalslandianfolded area: 5, gently folded Dalslandian formations; 6, Dalslandian granites. Precambrian igneous rocks, representing the time ofcratonization after the Svecokarelidic orogeny: 7, ; 8, Gothian granites and rapakivi granites; 9, volcanic rocks. Svecokarelidic folded area: 10, and gneisses; 11, plutonic rocks. Basement complexes: 12, Presvecokarelidic basement; 13, basement of unknown age (rejuvenated by the Dalslandian movements). Compiled by A. Simonen. the Precambrian geology in general. Further­ The oldest part of the Balnc Shield comprises more, many fundamental ideas on the the broad basement area in the east. Monot­ of metamorphic and granitic rocks are based onous, •cataclastic basement gneisses of on the studies carried out in different parts dioritic and granodioritic composition are the of the Baltic Shield. most abundant rocks of the basement area, where Geological Survey of. Finland, Bulletin 304 7 there occur only certain narrow schist zones, field). No valid evidence of a sedimentation composed mainly of mafic metavolcanics, meta­ floor or basement for Svecofennian sedimen­

or~s.ii.Tlle sediments and quartz-banded ration-has •.been found. Belornoridic gneiss belt and the arch The main phase of the Svecokarelidic orogeny;' of represent high-grade metamorphic which was connected with folding, regional. parts of the basement complex, which became metamorphism and plutonism, occurred about intensely reworked and recrystallized during 1 900-1 800 Ma ago. The synorogenic plutonic the younger Svecokarelidic orogenic movements. rocks, emplaced during the main deformation The age the basement gneisses is in most of phase of the Svecokarelides, are mainly quartz cases 2 600-2 800 Ma. However, even older , granodiorites and trondhjemites with ages, 3 000-3 500 Ma, have been sporadically minor amounts. of associated mafic rocks. The recorded expecially in the . late orogenic plutonic rocks are mostly potash­ In the southwestern part of the Baltic Shield, rich granites forming migrnatites with the older there occurs a basement complex of unknown rocks. Some small. postorogenic plutons (,....., age which was thoroughly reworked by Dals­ 1 800 Ma) mark the final stage of the Sveco­ landian tectonic movements about 1 000 Ma karelidic orogeny. ago. Some authors have correlated this basement to that of the eastern part of the Baltic Shield. The ·Svecokarelidic orogeny was followed by The main and central part of the Baltic Shield a long period of ' and cratonization. belongs to the area characterized by Sveco­ Numerous joints, faults and zones of karelidic folding. The geological evolution of highly varying ages produced a mosaic structure in the crust. Vertical block movements took the Svecokarelides has been a long one, and it shows many features similar to younger place and enormous masses of granites and orogenic belts. associated porphyries intruded into the stable The Svecokarelian sedimentation started with platform and sharply penetrated the Sveco­ an evolutionary, transgressive Jatulian group karelidic folded belt. To the platform intrusions, composed of basal beds, , dolornites representing the time of cratonization after the Svecokarelidic orogeny, .belong the rapakivi and sapropelic pelites. The Jatulian group of sediments was deposited about 2 200-2 000 .Ma granites, the so-called Gothian granites (e.g., ago upon the stabilized and deeply eroded the Smaland, Varrnland, Sorsele and Lina basement in the eastern part of the Baltic granites in ), their volcanic equivalents Shield. The Jatulian sedimentation is connected and diabases. The ages of the rapakivi granites with lively initial mafic volcanism, which took and Gothian granites vary between 1 550 and place at least in three different phases between 1 700 Ma. 2 200 and 2 000 Ma. The Jatulian rocks, which The deposition of the oldest red J otnian were metamorphosed in the Svecokarelidic -sediments, about 1 300-1 400 Ma ago, oc­ movements, either occur along the western rand curred upon the deeply eroded Svecokarelidic of the ancient or they form many sep­ rock crust. The Jotnian sediments are red arate basins upon the old basement. and tones and they form West of the ancient craton, mobile geo­ the oldest nonmetamorphic sedimentary cover synclinal belts developed and deposition of the of the Baltic Shield. The remains of the J otnian eugeosynclinal graywacke-basalt association scdiments only in deep down- took place. The total thickness of of the Svecokarelidic rock synclinal, Svecofennian strata is and sills occur abundantly meters (at least 8 kilometers in the deposits, and they represent 8 Geological Survey of Finland, Bulletin 304

hypabyssal eruption channels for basaltic magma merits were deposited, was situated only slight!y intrusions into the stable. platform. above the present surface. The Shield has had So-called Dalslandian or Sveco-Norwegian a tendency to lift, and the sediments of the tectonic movements took .place about 900­ platform cover are mostly eroded away or are 1 000 Ma ago in the southwestern corner of preserved in the downfaulted . The the Shield. The Dalslandian formations, de­ transgressions of younger geological times have posited on the ancient. basement. of unknown taken place only in the rand areas of the Shield. age; are of the platform type. and no geosynclinal The Baltic Shield was an uplifted peneplained strata are present. They have undergone a land area - approximately within its present gentle folding, and intrusions of plutons boundaries - until the glaciation. and pcgmatites have taken place. The .Dals­ The ice sheet covered the whole landian tectonic movements, connected. with Baltic Shield and its movements swept the old intrusions of. granites, have caused a regional clean, removing the loose material rejuvenation of the ancient basement in the and polishing the surfaces of the hard rocks. southwestern corner of the Baltic Shield. The Pleistocene glacial deposits consist mainly The youngest Precambrian of of till. However, stratified drift in ice marginal consist of so-called Eocambrian sediments, and radial deposits. are found all oyer •the which occur along the eastern rand zone of the Baltic Shield. Pleistocene and clay deposits ,ealedonides and are connected with the evol­ occur in the areas . formerly submerged by ution of the Caledonides. various stages of the . Remnants of the Paleozoic, Cambro- platform cover of the Baltic Shield are found The of the Baltic Shield took especially in the southern part of the Shield. place about 11 000-8 000 years ago, after These sediments were deposited on the ancient which extensive parts of the Shield were covered Sub-Cambrian peneplain, and were preserved by the Sea. The upheaval of the Shield area mostly in -like structures, which were started and continues at: present, being about formed in connection with Caledonian and one meter per century in the central part of the Hercynian movements. The afore-mentioned Shieldo tectonic movements caused deep fractures in The glaciation left clear marks on the geo­ the Shield area, along which alkaline magmas morphology of the. Baltic Shield, .which is intruded. The largest occurrences of Paleozoic characterized by the old peneplain with young igneous :rocks are in the Kola Peninsula and glaciation. Ice-polished of the Pre­ the Oslo graben. cambrian rocks, landscapes, , The Baltic Shield has been undergoing erosion deltas, glaciofluvial erosion forms, and thou­ for a long time, and the ancient Sub-Cambrian of lakes have created a landscape of peneplain, .on which the. oldest Paleozoic sedi- changing character and special beauty.

THE PRE,CAMBRIAN rr~FI]~LAND

In the following chapters, an attempt will be first, followed by a description of the geological made to give a short description of the main units and, in conclusion, remarks on their characteristics of the Precambrian in Finland. evolution. The economic geology of the country The geological and structural outlines, obtained is alsobrieHy summarized. bydiffere:nt methods of geoscience, are presented Geological Survey of Finland,. Bulletin 304 9

GEOLOGICAL AND STRUCTURAL OUTLINES

Our knowledge about the Precambrian in the. borders of the old basement. These schist Finland is based on investigations made by zones form the so-called Karelidic schist belt several generations of geologists. The results of theSvecokarelides which. extends from south­ of. systematic geological mapping and many eastern Finland to Lapland. The so-called Sveco­ regional geological studies are of fundamental fennidic schist belt of. southern and western importance in reconstructing a general picture Finland consists mainly of micaceous schists. of the evolution of the bedrock'. in .Finland. Both the Karelidic and Svecofennidic schists Systematic geological mapping of the. country belong to the Svecokarelides and represent has been carried on for about a hundred years different •types of sedimentary associations of by the Geological Survey-of Finland, which was the same orogenic cycle. The Svecokarelidic established in 1886. •The entire country· has plutonic rocks, consisting mainly of grano­ been surveyed: geological maps on the scales diorites and granites, ...are widely distributed of 1 : 200 000 (southern Finland) and 1 : 400000 and penetrate both the Karelidic and Sveco­ cover the total area.••Geological ••mapping in fennidic schists. more detail was begun at the end of the1940s. The youngest Finnish Precambrian granitoids The field investigations are recorded on· 1 : are represented by. many platform-type rapakivi 20 000 maps and the results are published as intrusions in southern Finland. The unmeta­ map sheets on the scale 1: 100000. Today, morphic, Precambrian sediments consist of so­ about 1/3 of the country has been .mapped called Jotnian arkose ••.sandstones and . on this scale. In addition to the geological So far as is known, the youngest Precambrian studies, especially the geochronological.· and of Finland is represented by dikes geophysical investigations have substantially cutting. the Jotnian .sediments. added to our knowledge about the evolution and structure of the Precambrian crust in Fin__ Structural. data land. The following geological and structural outlines have been prepared to summarize the The main structural units of the Finnish existing lithological, structural, geochronol­ Precambrian are given in Fig. 3. These units ogical, seismological, gravimetric, and aero­ differ from each other. in. age, , meta­ magnetic data on the Precambrian in Finland. morphism and structure. The oldest part (2600-2 800 Ma) is the so­ called Presvecokarelidic basement, which con­ Lithological data sists of some schist and gneiss zones, a granulite The lithological features of the Finnish bed­ arch, and ·wide areas of granitoid orthogneisses. rock are represented in Fig. 2, which shows The schists in this complex are penetrated by that the crust consists mainly of plutonic and granitoid.rocks of the age group 2 600-2 800 metamorphic rocks. Broad areas of ortho­ Ma.:· The granulite arch, a high-grade meta­ gneisses in eastern and northern Finland belong morphic part of the basement complex, was to the Early Precambrian or base­ thoroughly reworked by younger, Svecokarelidic ment. movements. The rocks included in the so-called Sveco­ The main. part of the. Finnish Precambrian karelidic folding are, however, most extensively h~lo11gs·~Q/the Svecokarelidic orogenic belt, spread. The schist zones, which are characterized the folding '.of which took place in connection by a rich abundance of quartzites, occur along with regional metamorphism and plutonism

2 127902042A 10 Geological Survey. of. Finland, Bulletin ··304

2 3 ~Jlit~ ~ 6 __

7

8

9 ~

10 .. ·····" .. EJ...... 11 12 ea 13 III 14 [[)]Ill]

too IpO

Fig. 2. Lithological map of the Precambrian in Finland. 1, orthogneiss; 2, granulite; 3, quartz­ schist; 4,. gneiss and migrnatite; 5, phyllite and mica schist; 6, ; 7, meta­ basalt and ; 8, , and ultramafic rocks; 9, granodiorite and quartz ; 10, granite; 11, ; 12, nonmetamorphic sedimentary rocks; 13, diabase; 14, Caledonidic schists. Drawn by Toini Mikkola after the map compiled by A. Simonen, Geological Survey of Finland, Bulletin 304 11

la - Ib - le ~ 2

-~----~ ---- 3

65 4: 65' ~ 5 m 6

60"

100

20° 25° 30'

Fig. 3. Main structural units of thePr.e<::~.1J:l.bri~pi!lFi!ll~nd.Presvecokarelidic: la, schist and paragneiss; lb, granulite; le, orthogneiss. svecokarelidic:2,.i Karelidic schist belt; 3, Svecofennidic schist belt; 4, orogenic plutonic rocks. Postsvecokarelian: 5,ntpakivi granites; 6, Jotnian sedi- ments. After .·A. Simonen. 12

about 1 900-1 800 Ma ago. The Svecokarelidic of the Finnish Precambrian, The ten age groups schists are divided into the. Karelidic and Sveco­ marked on the diagram are listed in thefollowing fennidic schist belts, which' deviate from .each with their geological interpretations:

other mainly in their lithological •••features, as Group 1 (2 800-3 000 Ma). The zircons of described in the foregoing.TheJ<~arelidicthis oldest age group have been found sporadi­ schists, with their rich abundance of quartzites, cally in> Presvecokarelidic .rocks. The oldest (~ are formations deposited upon.'••.•••or along the zircons .•3 000 Ma). occur as detrital' particles rands of the ancient basement. TheSveco­ in the Presvecokarelidic quartzite. fennidic schist belt consists mainly.ofmicaceous Group 2 (2600-2 800 Ma) comprises the schists and ... which produce a Presvecokarelidicrocks in the broad areas. of graywacke-basalt association-jof>the geosYtlc1inal the orthogneisses or so-called basement gneisses

sequence. Great masses ••,oforogenic .....plutonic in '.eastern' and northern Finland.. The basement rocks were emplaced into the schist belts during gneisses, which occur. as mantled gneiss domes

the Svecokarelidic orogenicmovements .. in the Svecokarelidic ••folded area, have been The Postsvecokarelian evolution of the Finn­ intensely reworked by later movements, and ish Precarnbrian is represented by. the intrusions their zircons give younger ages .( ~2 400­ of rapakivi granites (1 700__1.550 Ma) •and the 2500 Ma). sedimentation of nonmetamorphic,Jotnirnsedi­ Group 3 (~'2 500 Ma) consists of Presveco­ ments (1 300-1 400 Ma) . Furthermore,.to ••.this karelidic orthogneisses in northern Finland. evolution belong the intrusionsr.ofr.diabase Group 4 (2 430-2 450 Ma) comprises layered dikes, which are partlyconnectediwith .. the mafic intrusions, connected-with acid porphyries, rapakivi intrusions or are to some extent younger in northern Finland.• These differentiated, ­ than the Jotnian sediments. or lopolith-like intrusions penetrate the Pre­ svecokarelidic basement gneisses, but they are Geochronological data older than ·the basal. beds of the Svecokarelides. Group 5 (2 000-2 300 Ma) represents the

The frequency •diagram of the •.age ••deter­ evolutionary phase of the Svecokarelidic orogeny minations, reproduced in Fig. 4, gives an idea during which, the deposition of an epicontinental, of the main phases of the long. geological history transgressive Jatulian. group of sediments took

10 9 8 5 2

1 . .--{.J...... L-..-----'--"-r-~~~~"--i-'--'--'-+~'-t~~-IfIIL-J- 1.0 '1.5 2.() 2.5 Fig. 4. Frequency diagram of the V-Pb agedeterminations on the Precambrian rocks of Finland. Compiled by 0 .. Kouvo, Geological. Survey of Finland, Bulletin 304 13 place upon the Presvecokarelidic basement. The km thick...Thicknesses slightly over 20 km are dating of this group has been done mainly by found in southeastern Finland and less than 15 using mafic volcanics and diabases (2 200­ km in the area around the 2 000 Ma) connected with the Jatulian sedi­ and in the Caledonidic folded belt. mentation. The basaltic layer (Fig. 5, 11) is in most Group 6 (1 900-2 000 Ma) is based mainly places 15-20 km. Thicknesses over 20 km on the ages of the old rocks from are found in the area around the Gulf of Bothnia northern Finland that were intensely re­ and measurements of less than 15 km have been crystallized during the granulite metamorphism. mostly made locally in southeastern Finland. Furthermore, certain Svecofennidic metavol­ ·The.depth of Moho (Fig. 5, Ill) in the Baltic canics and plutonic rocks (trondhjemites) be­ Shield is commonly 35-40 km. Values over long to this group. 40 km are found in the area of the Gulf of Group 7 (1 850-1 900 Ma) comprises the Bothnia and the thickness of the crust in south­ Svecofennidic metavolcanics and synorogenic eastern Finland may be locally less than 30 plutonic rocks, mainly quartz .diorites and km. The average thickness of the Earth's crust granodiorites. The time involved represents in Finland is 39 km. the main phase of the Svecokarelidic folding and regional ..metamorphism. Gravimetric data Group 8 (1 750-1 850 Ma) consists of late­ orogenic and postorogenic intrusions of the Gravity anomaly maps of Finland have been Svecokarelidic orogeny. compiled by the Geodetic Institute and new Group 9 (1 550-1 700 Ma) comprises the maps are under preparation. A simplified free intrusions of rapakivi granites and gabbro­ air gravity anomaly map of Finland is pre­ as well as the diabases connected sented in Fig.. 6. with them. The original map published in 1962 on the Group 10 « 1 500 Ma), the youngest group scale 1: 1 000 000 is based· on measurements of the Precambrian in Finland, is represented made more than 10 000 stations. In southern by> the Jotnian sediments (1 300-1 400 Ma) Finland, the distribution of stations is fairly and the Postjotnian diabases (1 270 Ma). even, with an average spacing of 5 km. In northern Finland, the measurements represent mainly profiles along the roads, and there are 5eism0logical data wide areas without gravimetric data between The crustal structure in Finland is studied them. Mainly Worden-type gravimeters have mainly by the Institute of Seismology at the been used. The interval between the isoanomaly University of Helsinki. Explosion-seismic and curves in the original map is 5 mgal. earthquake studies carried out in different Certain of the major geological units of the northern countries have given a rough general Precambrian in Finland are clearly seen in the idea about the structure of the Earth's crust in gravity anomaly map. A wide area, showing the Baltic Shield. The sketch maps, based on a negative anomaly, occurs around the region refraction-seismic methods, are presented in of the Gulf of Bothnia where the thickness of Fig. 5, and they show schernatically the thickness the Earth's crust is greater than in the sur­ of the sublayers and Earth's crust in different Marked 20-60 mgal, negative parts of . characteristic of the rapakivi The granitic layer (Fig. 5, I) in Finlandand southeastern and southwestern in the area of the Baltic Shield is usually 15 20 ...... L.L"" .... .L.LJ.'-' .... "", areas of J otnian sediments 14 Geological Survey of Finland,'.Bulletin 304

Fig. 5. The thicknesses of the sublayers and Earth's crust in Fennoscandia. I, granitic layer; 11, basaltic layer; Ill, Earth's crust. Thickness in km. After E.

are characterized by a 20-30 mgal negative Aeromagnetic data anomaly. Positive anomalies seem to be a typical feature of the areas of the oldPre­ .A§Y$t~maticaerogeophysical mapping of the svecokarelidic basement in eastern and northern whole country (including the magnetic, electro­ Finland. magnetic and. radiometric measurements) was Geological Survey of Finland, Bulletin 304 15

69°

_ < 40mgal

- 40- -20mgal

- 20 - Omgal

0- +20mgal

+ 20- +40mgal

> +40mgal

Fig. 6. Free air gravity anomalies in Finland. Simplified from the original map ~(1: 1 000 000) published by the Finnish Geodetic Institute.

carried out in the years 1951-1972 by the The measurement of the Earth's total magnetic

Geological Survey of Finland. Flights for. this fi~ld\\V"a~rp.adewith a fluxgate magnetometer, survey were made at an altitude of 150 meters 'The results of the aeromagnetic survey are along parallel lines spaced 400 meters apart. summarized on maps· drawn .on the scale 1: 16 Geological Survey of Finland; Bulletin 304

-65 o

20° J Fig. 7. ..Aeromagnetic anomaly map of Finland. Geological Survey of Finland, Department.

20 000. The interval between the isoanomaly is a photographic reduction. In many cases, curves for the magnetic total intensity is 20 y owing to the reduction, the isoanomaly curves on. these original maps, of which the aero­ of the original maps meet to form coherent magnetic map of Finland presented in Fig. 7 and dark anomaly areas. Geological Survey of Finland, Bulletin 304 17

Most of the schist zones are cha.racter'ized :by granitoidrocks .are ••usually••••0 bserved to lack coherent anomaly areas, the form of which marked .magnetic. anomalies, and they appear yields information about the structura~/f~st~j,?~.a~twhiteareasin Fig. 7. Furthermore, the Jotnian of the crust. Small and rounded anomalY'areas sediments are characterized by lack of magnetic are usually produced by mafic and ultramafic anomalies. bodies of plutonic rocks. The massifs of

CHARACTERISTICS OF THE GEOLOGICAL UNITS

The characteristics of the geological units of scription is devoted. to the Svecokarelides, which the Finnish Precambrian will be given in form the largest and the most essential part of chronological order from the oldest to. the the .Precambrian .in. Finland. The Postsveco­ youngest complexes, The description. begins karelian igneous and sedimentary rocks, which with the oldest, Presvecokarelidic crust,'. which did. not take part in the erogenic movements, acted as a floor for the deposition of the Sveco­ are treated last. karelian sediments. The main part of .•the de-

Presvecokarelidic basement

The Presvecokarelidic basement ••complexes are pressed.••between the .surrounding blocks occur in eastern and northern Finlandand they of the. basement gneisses.

granitoid rocks of quartz dioritic, granodioritic smallIenses of ultramafics •.(serpentinites, soap­ and granitic composition is characteristic of stones and. talc-chlorite •schists) and with minor these basement areas. Originally, these gneissose, beds .of a quartz-bandcdjron formation. Silicic

granitoid rocks of the basement areas were metavolcanics ••·•occur only sporadically. The known as granite gneiss or gneissose· granite. metavolcanics have. been originally both iavas However, the composition of most of these and pyroclastics, and> they contain relics of a gneisses is not granitic, and hence the neutral porphyritic.·and.·.amygdaloidal texture and ag­ term »basement gneiss» has been suggested. glomeratic and pillow lava structure. The meta­ Some separate, narrow schist zones occur morphism of the mafic metavolcanics grades penetrated and surrounded by granitoid rocks. from the conditions of the greenschist : The granulite complex of Lapland forms a into the amphibolite facies. Metasedimentsare unique part of the Presvecokarelidic crust, represented by impure meta-, phyllites recrystallized in the Svecokarelidic movements. and mica schists. Graphite-bearing black schists occur as thin intercalations in the metasedirnents Basement area in eastern Finland as well as in the metavolcanics. The majority of the basement gneisses are The oldest rocks are schists and paragneisses considered. to be. orthogneisses, which had been penetrated by orthogneisses. The .I..I.' .L'"'.I.,I,t..,..·.y~.1..L+y'.l-y·' H+~+.'-:±.~."."r plutonic rocks ranging in form the isoclinally folded schist from quartz diorites to granites. Suomussalmi, Kuhrno and Ilornantsi, are. even-grained or porphyritic.

3 127902042A IM G·eological Survey of Finland, Bulletin 304

o,

1:::41 1:-:12 Fig. 8. Mantled gneiss domes in the Kuopio area. 1, basement gneiss; 2, ; 3, quartzite; 4, amphibolite; 5, dolomite; 6, mica schist. Aftet W. Wilkman and J. Preston. rocks, which are usually clearly foliated. How- quartz and feldspar grains of cataclastic rocks ever, weakly orientated and massive

Fig. 9. Basement gneiss. Inarinjarvi, Kotkavuono. Photo E. Halme. 20 Geological Survey of Finland, Bulletin 304 gneisses, which are penetrated by orthogneisses. The orthogneisses of the basement area The mica gneisses are highly metamorphic (Fig. 9) are highly similar to those met with and aluminous gneisses containing , in eastern Finland. The gneissose, mostly , and . Glassy quartz~granodioritic, and in many cases cataclastic ites, hornblende gneisses and amphibolites orthogneisses and migmatitic gneisses are com­ occur, too. mon. The zircon ages of the orthogneisses Many Presvecokarelidic schist and gneiss ia-ngebetw·een ··th'elimits 2500-2 800 Ma. zones from the most northern part of Finnish Lapland occur as steep between the Granulite arch basement gneiss blocks or as a marginal zone of .the granulite belt. The schist zones with A special part of the basement area .of-Finnish an abundance of amphibolites are related to Lapland is the so-called granulite complex, those in the basement area of eastern Finland. which ·is composed of para- and orthogneisses The stratigraphical sequence from youngest to that were metamorphosed under higher PT­ oldest for the schists seems to be .as follows: conditions than the surrounding rock crust. - amphibolites, with some black schist in Typomorphic minerals of Finnish their upper part; are: , almandine-pyrope, hypersthene , orthoclase and quartz. - mica gneisses, or locally quartz-feldspar The most common rocks of the granulite gneisses, mica gneisses, hornblende-gneisses arch are garnet gneisses (Fig. 10), the mineral amphibolites as and randomly alternating associations of which are: garnet-quartz-feldspar beds and bands, magnetite-banded quartzites gneisses, garnet-cordierite gneisses, garnet-bio­ and calc-silicate gneisses: tite gneisses and garnet-biotite-plagioclase - quartzites and quartz-feldspar gneisses. gneisses. The chemical composition, varying

Fig. 10. Fine-grained garnet-quartz-feldspar gneiss of the granulite complex. Utsjoki, Rittatshobma. Photo E. Halme. Geological Survey of Finland, Bulletin 304 21 from arenaceous to argillaceous,. the indistinct The zircon ages of the fine-grained garnet­ banding of different layers and the presence of quartz-feldspar gneisses of the granulite com­ quartzitic and graphite-bearing banp.ssllggest plex .:are ~ 2 150 Ma and the zircons of the that these garnet gneisses are of a sedimentary garnet-cordierite gneisses are about 2 000­ origin. 2 040 Ma. The Presvecokarelidic rocks re-' Hypersthene gneisses occur sporadically as crystallized by the granulite metamorphism thin intercalations in the garnet gneisses. The have a new generation of zircon and only plutonic rocks of the granulite area are mainly remnants of the old zircon generation have charnockitic quartz diorites associated with been met with. Evidence of the earliest event minor bodies of pyroxene-bearing mafic and of the granulite metomorphism (~2 150 Ma) ultramafic rocks. There are also occurrences is preserved only in the southwestern part of of garnet-biotite quartz diorites and garnet­ the granulite arch. The later high -grade meta­ bearing porphyritic granites. morphism (~ 1 900 Ma) laid its stamp on the The metamorphic and tectonic evolution of entire area. It is noteworthy that the isotope the granulite complex is highly complicated composition of the. total lead of the whole rock and characterized by quite a thorough ·re­ corresponds, however, to that of the ancient working, which occurred during the Sveco­ Presvecokarelidic age group ~ 2 500 Ma. karelidic orogeny. The gneisses of the granulite For example, the ages of the gra,nulites in the arch are polymetamorphic rocks, which formed Koppeloarea (south of Lake Inari), influenced stable structures as a result of the Presveco­ by. a later phase of metamorphism, are as karelidic movements; but they took part· in follows: later tectonic movements of the Svecokarelides. zircon 1 940 Ma The granulites were formed from ancient Presvecokarelidic amphibolite-facies rocks by monazite 1 915 Ma kinetic metamorphism in the shear zones. The total lead of the whole rock granulite metamorphism (~ 2 150 Ma) was (isochron): 2 Ma followed about 1 900 Ma ago by later meta­ 465 morphism under conditions of low-grade granu­ lite facies and high-grade amphibolite facies. and the age data of the pyroxene quartz diorites As a consequence of the Svecokarelidic move­ in the granulite area as follows: ments, the granulite massif was uplifted and zircon 1 925 Ma overthrust towards the southwest, causing the Svecokarelidic schists to follow' the tectonics total lead of the whole rock of the granulite arch. (isochron) 2 535 Ma.

Presvecokarelian igneous rocks ..younger than the folded basement

Penetrating the Presvecokarelidic folded base­ Maftc igneous rocks ment, layered mafic intrusions represent the Presvecokarelian igneous rocks older than the Many differentiated and layered mafic in­

Svecokarelides. Furthermore, the i"\rt.i"' .... " ....J"'i'.i"\ Finland, which are usually this croup seem to include a unique cal~,lfjC)na,ttt'eSvecokarelian rocks, seem, how­ complex in ·Siilijarvi, which is one than the Svecokarelides, These carbonatite occurrences in the world. occur as sheets or lopolith-like bodies 22 Geological Survey of Finland, Bulletin 304

mm

B 2

1~13

• 4

5

Fig. 11. Presvecokarelian, layered mafic intrusions in northern Finland. 1, Caledonidic rocks; 2, Svecokarelidic plutonic rocks; 3, Svecokarelidic schists; 4, Presvecokarelian, layered mafic intrusions; 5, Presvecokarelidic basement (schists, basement gneisses and granulites). Geological Survey of Finland, Bulletin 304 23

in the Presvecokarelidic basement area or along relics of the original minerals are, however, the contact zones between the ancient basement present. Furthermore, the reworking produced and the Karelidic schist zones (Fig. 11). The a crushed texture and the contacts of the malic mode of occurrence and age data show that bodies were tectonized. the mafic intrusions had emplaced the stabilized The ages of the layered mafic intrusions were Presvecokarelidic basement before the evol­ determined by the V-Pb method for zircon. utionary phase Svecokarelides. of the The Zircon occurs in the coarse-grained, pegma­ differentiated mafic bodies may have originated toidic parts of the . The age determi­ as platform-type intrusions in the ancient crust nations show that the layered mafic rocks form between the Presvecokarelidic and Sveco­ a coherent age group of 2 430-2 450 Ma. karelidic folding movements. Study of these layered mafic intrusions is still in progress and their geological position and importance in the Carbonatite complex geological evolution of Finland are still an open question. In addition to the afore-mentioned mafic The chemical composition of the magma was intrusions, the peculiar carbonatite complex basaltic, tholeitic or ossipitic. High contents of Siilinjarvi may also belong to the group of of al umina and lime and very low contents of Presvecokarelian igneous rocks. This complex alkalies are characteristic. The differentiation forms an elongated, vertical sheet-like body, of the magma produced mainly peridotites, which penetrates the Presvecokarelidic basement gabbros, anorthositic gabbros and anorthosites. gneiss. The main rock types, in succession to The ultramafic differentiates form the lower the intrusions, are: glimmerite (phlogopite parts and the anorthositic types the upper' parts rock), and carbonatite. The fenitic of the differentiated bodies. Common features margins around the carbonatite complex are are gradual transitions between different varieties weakly developed. and rhythmic layering. Some of the mafic bodies Carbonatite was intruded into glimmerite, are associated with granophyres and silicic producing mixed rocks. Most common are porphyries in the uppermost parts of the glimmerite-carbonatite rocks, the carbonatite layered intrusions. content of which varies from 5 to 25 %. The The most common primary minerals of these main minerals of the glimmerite are phlogopite, mafic rocks are , orthopyroxene, augite alkali amphibole (richterite) and apatite. The and plagioclase. Owing to autometamorphism trace elements enriched in the carbonatite after magmatic crystallization and to reworking complex are P, F, Sr, Ba, RE (especially Ce caused by the Svecokarelidic movements, the group), Zr and Nb. The age of the complex mafic minerals have been altered into serpentine, is about 2 500 Ma, as determined by the K-Ar talc, uralite and chlorite, etc. Remnants and method for richterite.

Svecokarelides

The Svecokarelides cover most of the Finnish Finland, characterized especially by an abun­ Precambrian area, and the Svecokarelidic·folded dance of quartzites. zone is composed the of schist belt in western and geological units (cf., Figs. 2-3): U'-',I".I. ... .l..J.."-'.L.l..l. Finland, characterized by an abun­ - Karelidic schist belt in eastern and northern dance of micaceous schists. · 24 Geological Survey of Finland, Bulletin 304

- Orogenic Svecokarelidic plutonic rocks, em­ preserved sedimentary structures, such as ripple placed during the Svecokarelidic movements marks, cross bedding, and mud cracks, are and penetrating the Karelidic and Sveco­ found in low-metamorphic, blastoclastic quartz­ fennidic schists. ites. The micaceous schists show graded bedding and stratification. Porphyritic, amygdaloidal, pillow lava and agglomerate textures and struc­ Karelidic schist belt tures are common in metavolcanics, The metamorphism of the Karelidic schists The main rock types in the Karelidic schist took place in the conditions of the greenschist, belt, the trend of which is from southeastern to -amphibolite or amphibolite facies. Mi­ northern Finland, occur in the following pro­ caceous schists grade from, phyllites into mica portions: schists and mica gneisses. The mica schists

micaceous schists . 45.2 % contain porphyro blasts ~ofandalusite, cordierite quartz-feldspar schists . 2.8 % or staurolite. Migmatic mica gneisses .contain quartzites . 26.4 % garnet" cordierite and sillimanite, The meta­ limestones . 0.3 % basalts 'of northern Finland are commonly metabasaltsand amphibolites . 25.3 % greenstones, consisting of albite, epidote, and chlorite. The greenstones grade into amphibo­ Originally, the Karelidic schists were mainly lites, especially along the contact of plutonic argillaceous sediments and true quartz ­ masses. stones, whieh metamorphosed into phyllites The folding of the Karelidic belt took place or mica schists, and quartzites. Metavolcanics ­ against the Presvecokarelidic basement blocks, originally mafic lavas, tuffs and diabases - are and gentle synclines and developed. also corn.mon. The widely distributed meta­ The axes are gentle and linear structures volcanics of northern Finland are associated commonly run parallel to them. Axial cul­ with ultramafic, picritic bodies and with beds minations and depressions are typical ,of the of jaspilites. The limestones, are mainly dolo­ Karelidic belt of eastern Finland, where the mitic. Further, there .are minor occurrences of ancient basement or the floor of the many other rock types, including conglom­ appears in the culminations and anticlines of erates, kaoline deposits, arkosites, carbonaceous the folded belt. Sections through the Karelides, black schists, etc. showing some examples of the folding, are The metasediments and metavolcanics of the presented in Figs. 12-13. Karelidic schist belt show, as a' relic fabric, The standard stratigraphical section of the many primary textures and structures. Well- Karelidic metasediments, especially in eastern

MAAI~IANVAARA + __ ------" _, KONTIOL~~!J_

i + :j~~m~:'~;"':;;1f~ltWJlltf'IJfiij;,:j,~~;;:;;~"":~'M::;:::!:::;;:g;,;:))))i!!li~;{ i

\:r~r.~·:~v~;13 .11 2 10 km Wegmann 1928

Fig. 12" Section through the Karelides in' North Karelia, E-Finland. 1, Presvecokarelidic basement gneiss; 2, basal quartzite; 3, quartzite; 4, phyllite and mica schist; 5, ophiolite (serpentinite); 6, granitoid. After E. Wegmann. Geological Survey of Finland, Bulletin ·304 25

00 ~9 .....10 0 2 _3 c::::J4 //

Fig. 13. Section through the Karelidic schist zone in the Rukatunturi area, Kuusamo, 1, Presvecokarelidic basement; 2, basal beds of the Karelian strata; 3,. greenstone; 4, sericite quartzite, sericite schist and quartzite; 5, siltsized argillaceous and dolornitic schist; 6, sericite quartzite and orthoquartzite; 7, dolomite; 8, amphibole schist with black schist intercalations; 9, subsilicic intrusives, mainly sills; 10, fault. Compiled by A. Silvennoinen,

Finland, which has been a key area in the formation has been called the Kainuian or interpretation and classification of the Finnish Jatulian quartzite formation and it serves as an Karelides, is as follows: excellent key bed for stratigraphical correlations. Kalevian group: phyllites and mica schists The quartzites are overlain by so-called marine Iatulian metasediments: dolornites and car­ carbonaceous slates bonaceous slates, associated in some cases with hematite-bearing iron formation, The meta­ Jatulian group: quartzites sediments of the J atulian group represent a basal conglomerates and transgressive epicontinental sequence upon the arkoses peneplained Presvecokarelidic basement. The Great unconformity time of the Jatulian sedimentation is associated Presvecokarelidic basement with contemporaneous volcanisrn represented by metabasalts and metadiabases, which are The Karelidic schists are separated by a present in all the areas consisting of the meta­ marked unconformity from the Presvecokarelidic sediments of the Jatulian group. Metadiabases basement (Fig. 14). The Jatulian sedimentation penetrate both the basement and the Jatulian started with basal arkoses and conglomerates sediments, but not the Kalevian rocks over­ lying upon the Presvecokarelidic basement, Iying the Jatulian. The mafic volcanism inter­ which sometimes shows relicts of the crust rupted the Jatulian sedimentation. Usually, one weathered in situ, to a depth of some ten meters. or more interbeds of mafic metavolcanics are The basal arkose, with intercalations of silts tone, found in the Jatulian strata. The beds of meta­ calcareous and quartzitic beds, overlies the volcanics in some cases directly overlie the weathered surface of the basement gneiss. The ancient basement. basal polymict conglomerates (Fig. 15), which The J atulian transgressive group, the total contain pebbles of Presvecokarelidic gneisses, thickness of which is usually some hundreds occur only sporadically as basal beds of the of meters, is overlain by many kilometers of Karelian strata. This basal formation has been thickly accumalated, so-called I

Oy~r:l~RATt.lt.R uranium deposits rest upon the Sarioli411..ba.sa1 .•Jatulian sediments unconforrnably beds or directly upon the Presvecokarelidic and conglomerates, containing pebbles of the: basement. This widely distributed quartzite basementgneisses and Jatulian metasediments.

4 127902042A 26 Geological Survey. of Finland, Bulletin 304

Sark\ \amp',

A B

o 100 200 300 m I I I I

') v:t'Fl:1:r:J~.) 1­ . 4.. IxXXX x \ 3 Fig. 14. Basal conglomerate in Polkkylampi, Kiihtelysvaara, •E-Finland. 1, Presvecokarelidic orthogll~iss;2,Pr(;sve<:()~~reliclifpa:ragneiss;3, tourmaline­ bearing granite; 4, polymict conglomerate (containing pebbles of the rocks' 1-3);.5, quartzite. After B. Frosterusand W. Wilkman. Geological Survey of Finland, Bulletin 304 27

o 5 10 15 I I I ! Fig. 15. Basal conglomerate, Viesirnonjoki, Kiihtelysvaara. Photo M. Harme. and metavolcanics, separate the groups from Table 1. Stratigraphy of the Rukatunturi area, after A. Silvennoinen. each other. The marked unconformity between Thickness in meters the Jatulian and Kalevian successions does not, Formations S-part N-part however, exist in all sections of the Karelian amphibole schist . >250 sedimentation area. dolomite . ? 1 50-100 I An extremely thick sequence of the Jatulian Rukatunturi quartzite . 800 ?600 greenstone III . 200 200 strata occurs in the Kuusamo area, where the . 200 200 greenstone 11 . 50 o ]atulian transgression is interrupted by three quartzite schist . 50 o stages of basic volcanism and slight regression sericiteschist . 130 sericite quartzite . 200 I ?100 after the second phase of volcanism.The greenston~iI.; . ?500 stratigraphy of the Rukatunturi area in Kuusarno basal conglomerate . 0-20 -- unconformity -­ is given in Table 1. Presvecokarelidic crust .... I 28 Geological Survey of Finland.. Bulletin 304

(~. The I

Karelidic schists." This. younger •..seditnentary micaceous. schists. and gneisses •••....•... 79.9 0/0 formation is called the I

formation may be' sedimentsvof the' :molasse inally.••mainly mafic .•lavas •and pyroclastics, are type. The metavolcanics •.,and. associated meta­ common' in •some. •Svecofennidic schist areas. diabases represent initial volcanism of the Furthermore, there are small quantities of many

Svecokarelides, and. their ••rich .occurrence' as other .•rock types, such 'as,' for example intra­ intercalations and' sills in. the ••Jatulian. group formational conglomerates.. calcareous schists, indicates marked tectonic activity, characterized black schists, etc. Relics of the primary textures by deep fractures, along the border zone be­ and structures of the Svecofennidic metasedi­ tween the stable Presvecokarelidic craton and ments and metavolcanics have been found only the .developing geosyncline. in certain of the best-preserved parts of the The age of the Jatulian magmatism (meta­ schist belt; but in wide areas of markedly re­ volcanics and metadiabases) ranges between the crystallized gneisses and migrnatites, they have limits of 2200-2 000 J\1a, dated by the U-Pb been destroyed. method on zircons. The Pb-Pb age of the dolo­ Micaceous schists are represented by gray­ mite and the age (total lead" of the whole rock, wacke slates, mica schists and mica gneisses. isochron) of the iron formation, both of which The graywacke slates show graded bedding rocks belong to the upper part of the J atulian (Fig. 16) and a blastoclastic texture. They pass group, are ~,2 050 Ma and ~ 2 080. Ma, gradually into granoblastic mica schists, which respectively. On the basis of these ages, it may show only sporadically relics of graded bedding. be concluded that the time of the J atulian The intensely recrystallizedmica gneisses, which sedimentation extended over 200 Ma (r~2 200 are' widely distributed, show only indistinct -2 000 Ma ago). The Kalevian sediments, banding C)fdifferent layers, owing to the primary which overlie the J atulian, are younger than variation in the sedimentary strata. The afore­ ~ 2 000 Ma, but older than the synorogenic mentioned micaceous. schists, which are the Geological Survey of Finland, Bulletin 304 29

Fig. 16. Graywacke-slate with graded bedding. Siivikkala, Ylojarvi. Photo M. Harme,

most abundant metamorphites of the Sveco­ texture and structure, likewise indicate a volcanic fennidic belt, have been interpreted as prod­ origin. The pyroclastic material shows relics ucts of incomplete ; and they of banding of different layers and beds of contain both argillaceous and arenaceous ..ma­ agglomerates, and volcanic conglomerates afford terial. They are similar to the graywackes of the best evidence of a volcanic origin. younger orogenic belts. The Svecofennidic schists had been usually The quartz-feldspar schists are by and large metamorphosed under conditions of low­ layered, and relics of cross-bedding have been pressure amphibolite facies, but in some places found. Most of the quartz-feldspar schists, under conditions of the granulite facies. In some of which are mixed with argillaceous and certain narrow zones, retrograde metamorphism calcareous material, are interpreted to be mainly took place producing mineral assemblages of arkose sandstones; but some of these show the greenschist facies. chemical as well as textural characteristics of The graywacke slates with well-preserved silicic lavas and pyroclastics. sedimentary structures represent the lowest The metavolcanics, which are mainly basaltic grade of metamorphism of the Svecofennidic or andesitic in chemical composition, having micaceous schists. They do not contain por­ been crystallized from the lava flows, are phyroblasts of AI-minerals and they occur in usually seen to exhibit as a relic a blastopor­ the schist zones where penetrating plutonic phyritic texture; but other relic fabrics, such bodies'. are rare. The graywacke slates pass as fluidal, amygdaloidal, perlite, and pillow lava gradually into mica schists, usually seen to 30 Geological Survey of Finland, Bulletin 304

N

[2] _3_2

r.".:·.·)·:.«14 5Am ~5

Fig. 17. The Tampere schist belt with vertical sections. 1, plutonic rocks; 2, rnafic metavolcanics; 3, congJlomerates; 4, quartz-feldspar rocks; 5, graywacke-slates. Arrows show the direction of the base determined by graded bedding. After A. Simonen. contain porphyro blasts of cordierite and The mafic metavolcanics usually show the andalusite and in some cases staurolite. The mineral association hornblende-plagioclase, highest grade of metamorphism is represented which indicates the conditions of the amphibolite by coarse-grained mica gneisses containing facies. In certain narrow zones, owing to retro­ almandine, cordierite and sillimanite. gressive metamorphism, the mineral associations Geological Survey of Finland, Bulletin 304 31

~~2-- b-~':1'4 Fig. 18. Cross section of the Parainen () basin, southwestern Finland. 1, garnet-bearing mica gneiss; 2, amphibolite; 3, limestone; 4, granite with remnants of garnet-bearing mica gneiss. After A. Metzger. of the arnphibolite facies were altered into those zones are markedly curved. The attitude of of the epidote-amphibolite and greenschist the schistosity and is usually parallel facies. The associations produced have not to that of the bedding. A schistosity transverse attained complete equilibrium, but many un­ to the bedding schistosity occurs only sproradi­ stable relics of higher-grade regional meta­ cally as a fracture cleavage and as axial plane morphism are common. schistosity. Furthermore, foliation caused by The charnockitic rocks of the granulite facies faulting occurs along the numerous fault zones occur in the West Uusimaa complex, southern of different ages. Finland, where the association hypersthene­ The Svecofennidic schists are conspicuously diopside is typical. In the mica gneisses of this folded. The fold axes of the main folding are area, garnet and potassium feldspar occur gentle or subhorizontal.The axial planes are together with cordierite, steep .or vertical. The synclines and anticlines The general strike of the Svecofennidic belt have. often been tilted into steep isoclines. The is quite variable. Rectilinear schistosity" is type of folding varies with the plasticity of typical in the zones of abundant metavolcanics, the folded material. The micaceous schists have but broad areas of the migmatitic mica gneisses show great local' variations in the strike of the been deformed into small folds, whereas the foliation. The dips of the schistosity and the more competent beds of the metavolcanics may foliation are usually vertical or synclines, Sections through the dipping foliation occurs in the areas showing some examples of the matitic gneisses, where the trends of presented in Figs. 17-18. 32 Geological Survey of Finland, Bulletin 304

The direction of the lineation, appearing as calcareous material and an elongation of the minerals and the mineral mature sandstones in the aggregates, or as crenulation and small folds, arkoses of some areas. varies greatly in relation to the subhorizontal tectonic b-axis of the major folds. In the areas The sequence of the 'Tampere schist zone, of abundant competent beds of metavolcanics, which has been a key area in working out the the linear structures observed on the nearly stratigraphy, is from the highest stratigraphical vertical bedding planes are generally quite unit to the lowest as follows: steep and run in the direction of. the tectonic Thickness in meters transport, perpendicular to the subhorizontal mafic volcanics . ,> 1 000 b-axis of the main folding. Moderate and gently conglomerates and associated dipping lineations are typical features in the beds of graywackeslates and areas of silicic schists and. mica gneisses and arkoses ...... •...... 700- 800 they often show parallelism with the fold mafic and intermediate vol- axis. The updoming of the migmatite granite canics 800-1 500 bodies of the migmatite terrains has caused quartz-feldspar rocks (arkoses, many complications in the directions of the grayvvackes and pyroclastics) 1 500-2200 lineations, and faults of different ages have gra ywacke slates .. . >3000 caused lineations showing varying directions. An interpretation of the stratigraphical se­ This sequence includes the Lower and Middle quence of the intensely folded and migmatitic Svecofennian subgroups. The total thickness Svecofennidic schist belt has been possible of the strata, which are related to the gray­ only in certain key areas, where the structures wacke-basalt association of eugeosynclinal de­ are simple or the relics of the sedimentary or posits, is at least 8 kilorneters. volcanic structures are so. well preserved as Taken as a whole, the Svecofennidic schists to allow the sequence of layered rocks to be were originally products of a geosynclinal determined, although the beds have been folded sedimentation. The micaceous schists show to a vertical position. compositional and structural characteristics In many Svecofennidic schist zones, thick ac­ similar to the graywackes of the orogenic belts. cumulations of immature metasediments underlie The quartz-feldspar schists have been inter­ the mafic metavolcanics, which in some areas preted mainly to be impure arkoses. The mafic are overlain by micaceous schists. The Sveco­ metavolcanics represent geosynclinal volcanism fennian strata have been tentatively divided and their intimate association with immature into Lower, Middle and Upper Svecofennian sediments shows the eugeosynclinal character subgroups as follows: of the Svecofennidic schist belt. Upper Svecofennian: argillaceous sediments The data illustrating the age of the Sveco­ fennian sedimentation are very scanty. The age Middle Svecofennian: mafic volcanics (lavas of detrital zircon from the metagraywacke in and pyroclastics) with the Tarnpere field is ....,....2 400 Ma, indicating the intercalated sediments minimum age of the .pre-existing source area. (arkoses, graywackes On the other hand, the intraformational con­ and conglomerates) glomerates of the Tampere field contain some Lower Svecofennian: immature sediments pebbles of plutonic rocks, whose zircon is only

(arkoses and gray"wackes) ....,....1 900 Ma or highly similar to that of the

with thin interbeds of granitoids (-...~1 880.Ma) penetrating the schists. Geological Survey of Finland, Bulletin 304 33

It is only possible to state tentatively that the Plutonic rocks of the Svecokarelides Svecofennian sedimentation took place about 2 400-1 900 Ma ago. Presumably, however, Abundant plutonism, characterized especially the geosynclinal deposition happened for the by the emplacement of the granitoid rocks, most part immediately before the climax of is a typical feature of the Svecokarelidic orogeny. the orogenic movements, which were marked The plutonic rocks occupy about 40 Cj~of the by intrusions of synoro genic granitoids with I<.arelidic folded area and about 80 % of the an age of ...... 1 900 Ma. The Svecofennian geo­ Svecofennidic area. The abundance of the synclinal volcanism has been dated using the different plutonic rock types in the Karelidic zircon fractions of intermediate and silicic (1) and Svecofennidic (2) folded. areas is as metavolcanics. The ages range between the limits 1 920-1 880 Ma. It is particularly noteworthy follows: that the Svecofennian geosynclinal volcanism 1 2 is younger than the Jatulian volcanism (2 200­ ultramafic and gabbro .... 3.3 0/0 5.9 0/0 2 000 Ma) and only slightly older than the quartz diorite and grano- synorogenic plutonism of the Svecokarelides diorite . 23.6 % 56.5 % (1 880 Ma). granite 73.1 0/0 37.6 %

10 20 Km ! !

Fig. 19. The zones of the ultramaficsinthe()utokumpu field, North Karelia. 1, Presvecokarelidic rocks; 2, Karelidic rocks; 3, bedding; 4, zone of ultramafiqs (Outokumpu zone); 5, sulphide . After A. Huhma and E. Peltola. 34 Geological Survey of. Finland, Bulletin 304

The occurrence of ultramafic small bodies, characteristics. In the Svecofennidic area, the metamorphosed and metasomatically altered plutonic rocks of which are studied in more into •serpentinites, talc-magnesite rocks and detail, the petrographic provinces have been chlorite-, - or asbest-bearing rocks, named according. to the most acid end members is a characteristic feature of the Kalevian group as follows: of the Karelidic schist belt. These ultramafics granodiorite province, occur as lenticular bodies forming discontinuous trondhjemite province, zones as interbeds in the Karelidic micaceous province, schists (Fig. 19). They are mainly antigorite­ granite province, and and chrysotile-bearing serpentinites. Relics of microcline granites. olivine indicate that the original rock was a dunite, A characteristic feature of these ultra­ Granites are abundant among the plutonic mafics is the complete absence of more silicic rocks of the Karelidic belt. Some of these are differentiates. the most silicic members of continuous rock Ultramafics of the .Svecokarelides, associated series containing mafic and intermediate rock with silicic differentiates, are represented by types. These rocks show similarities to the small bodies of peridotites or hornblendites, plutonic rocks of the granite province. Some of Peridotites usually contain serpcntinizcd olivine, the granites are migmatite-forming and related diopsidic pyroxene and hornblende. These to the microcline granites. ultramafics occur commonly in connection with Variations in the mineralogical composition gabbro bodies and pass gradually into the more of different plutonic .rock provinces are given silicic rock types. Most of the gabbros contain in Fig. 20. 110 the separate provinces, excepting hornblende and relics of diopsidic pyroxene the microcline granitys, belong mafic, inter­ surrounded by hornblende have only oc­ mediate, and silicic plutonic rocks, which form casionally been found. Noritic gabbros, con­ continuous rock series suggesting magmatic taining both rhornbic and monoclinic pyroxene, differentiation. Furthermore, reaction series of occur in association with pyroxene-bearing the mafic and salic minerals of the provinces silicic differentiates. The main salic mineral indicate magmatic evolution. The most silicic of the gabbros is a plagioclase that shows end members of the granodiorite, trondhjemite zoning with a more anorthitic core. Anortho­ and charnockite provinces are characterized by sites and anorthositic gabbros are rare. A a preponderance of plagioclase over potassium striking feature of the gabbros, with a color­ feldspar, whereas real granites (Or> Ab --1-An) index of 30-50, is that they contain small are present only in the granite province and amounts, 10-15 %, of free quartz. These among the migmatite-forming microcline gran­ quartz gabbros are quite common among the ites. Svecokarelidic gabbroidic rocks. The provinces characterized by plagioclase­ The majority of the Svecofennidic plutonic rich silicic end members differ from each other rocks are represented by different kinds of mainly in respect to the different reaction types granitoid rocks whose color-index is < 30. of mafic minerals. A meta-aluminous association These rocks are commonly quartz dioritic, (hornblende and biotite) is characteristic of most granodioritic or granitic in chemical com­ of the silicic members. of the granod'iorite prov­ position. The plutonic rocks are calcic or ince' a peraluminousreaction type (biotite) of calc-alkalic, and detailed petrographic study has the trondhjemite province, and a subaluminous shown that they form different plutonic prov­ reaction type (pyroxene) of the charnockite inces with their own mineralogical and chemical province. Differences in the mineralogical com- Geological Survey of Finland, Bulletin 304 35

m··· ············n·...... ····.··....······ ...... x·.·.· · ••. .0 .• . 80 ~~ x·. • 40

20

40 50 60 70 40 50 60 70 40 50 60 70 ------Jli.,.__ 5 i 0 2 % .. . e .....•••••• oN u····~ .: ...... [2J 1 x BD ~rx?7l 2 ~ x x x 3 60 x x xxx x 4 x x x x x ,••A£.&&15 40 :

. .•. 20 ~ ~••••• 7 ~<. 8

~o 50 60 70 • 60 70 Fig. 20. Mineralogical composition of the plutonic rock provinces. I, granodiorite province; 11, trond­ hjernite province; Ill, charnockite province; IV, granite province; V, microcline granites. 1, quartz; 2, microcline; 3, plagioclase; 4, biotite; 5, hornblende; 6, pyroxene; 7, olivine; 8, accessories. After A. Simonen, positron of the provinces mentioned are most the other provinces, the trondhjemites are apparent in silicic end members, whereas the characterized by a high content of soda; and mafic rocks of different provinces are mineral­ a high content of potassium is typical both of ogically closely related. the granite province and of the microcline In addition to the mineralogical differences granites. between the plutonic provinces there are also The chemical characteristics of the plutonic chemical differences, which appear especially provinces are given by normative proportions in the content of CaO, Na20 and 1<20. The (Fig. 21) and Q: Ab : Or (Fig. rocks of the granodiorite province contain fields of the trondhjemite and char­ more lime than the corresponding rocks of provinces have the same position, and 36 Geological Survey of Finland, Bulletin .304

Or

1 ------2 ------3 70 -'-._._.-.- 4 5

50 50

70 30 Fig. 21. Normative proportions Or : Ab : An of the plutonic provinces. 1, grano­ diorite province; 2, trondhjemite province; 90 3, charnockite province; 4, granite prov­ ince; 5, microcline granite. After A. -'-. Simonen. 10 30 50 70 90

Q

/------21 ------3 _.-.- 4 ...... 5 70 §lllJ 6

50

30 Fig. 22. Normative proportions Q: Ab : Or of the plutonic provinces. 1, grano­ diorite province; 2, trondhjemite province; 10 3, charnockite province; 4, granite prov­ ince; 5, microcline granite; 6, »eutectoid» granite field. After A. Simonen. 10 30 50 70 90 they deviate distinctly, especially by having a Chemical differences between the plutonic high content of Ab, from the other provinces. provinces are shown also by the alkali-lime The granodiorite, trondhjemite and charnockite index (Fig. 23). The granodiorite province is provinces do not continue into the »granite the most calcic with an index of 62, the trond­ held». The most silicic members of the granite hjemite and charnockite provinces have an province show a eutectoid granite composition, index of 60-61, whereas the granite province whereas the microcline granites are characterized is calc-alkalic, with an index of 59. by a higher content of Or than the granites The magmatic origin of plutonic rocks of the granite province. belonging to provinces with a gradual transition Geological Survey of Finland, Bulletin 304 37

'"-, -----1 CeNl ----- 2 10io X ---e_e- 3 X~

~.--- ""~-~ /- ..~ / -~ .i--:>:- ---. ---- 5%

Fig. 23. Alkali-lime /index of the plutonic provinces. 1, granodiorite province; ~,trondhjemite province; 3, granite province. After. A. 50 60 70 Simonen.

from mafic into silicic members has been karelidic plutonic .rocks.• These granites .do not pointed out by various authors .. The chemical belong to any..ofthe other plutonic provinces, characteristics of the plutonic provinces indicate as a silicic end member, because a marked slight regional differences in the .composition break occurs between them and the other of the parent magma. The primary compositional provinces. These granites are not genetically differences, appearing especially in the content of associated •with mafic and intermediate rock CaO, Na 20 and K 20, have been the principal types, but on the contrary they show much factors producing different petrographic prov­ evidence of metasomatic granitization. The inces. The main factor controlling the dif­ granitization of all kinds of primary material ferent paths of evolution of the trondhjemite (schists and plutonic rocks) tends toward a and charnockite provinces, which are chemically granitic composition of the microcline granite, closely related, has been the different contents which does not correspond to a »eutectoid» of volatile substances in the parent magma. granite composition of. magmatic evolution. The trondhjemites have been differentiated from The granite formed by. granitization usually a parent magma rich in water, whereas. the contains ghost-like, nebulitic remnants of the have been crystallized from a dry pre-existing rocks. The relic stratigr~phyand magma. The preponderance of silicic plutonic structures of many microcline granite bodies rocks over gabbros suggests that the parent indicate that in the place of the present granite magma had not been basaltic, but more acid, there had been older solid rocks, and the probably quartz dioritic o~ granodioritic in granite body has been emplaced by meta.. composition. Great quantities of acid, anatectic somatic processes. magma may have originated in the root. zone A •.characteristic feature of many Sveco- of the ancient mountain chain. kftfB!ici!c:areas, especially in southern Finland The migmatite-forming microcline granites and around the wide granite massif of Central have a unique position among the Sveco.. Lapland, ..is.: an abundance of mixed rocks or 38 Geological Survey of Finland, Bulletin 304

Fig. 24. Some structures of the Svecokarelidic . 1, agmatite; 2, veined gneiss; 3, nebulite; 4, poly­ migmatite. Drawn by Toini Mikkola.

migrnatites with two components of different which are granites showing only a relic stra­ character. The older component is a meta­ tigraphy and structure of the primary host morphic schist or a plutonic rock and the rocks of the migmatite granite. The large younger component is an igneous-looking bodies of migrnatite granites are the final prod­ granitoid rock, usually :microcline-rich granite. ucts of migmatization and granitization. The The migmatite-forming microcline granites with origin of the large nebulitic granite masses associated and aplites penetrate all signifies that great quantities of granite material the older rocks, forming a great variety of had moved into a zone of regional migmatization migmatites, some typical of which structures and granitization. This material had probably are given in Fig. 24. been a granite melt, which caused the meta­ Migmatization caused by the emplacement of somatic granitization of the host rocks. microcline granite is closely connected with The emplacement of the Svecokarelidic plu­ the metasomatic granitization of the host rocks, tonic rocks is closely connected with the and gradual transitions between the different orogenic, mountain building movements. The components of the migrnatite are therefore commonly used tectonic classification of the common. Because of intense metasomatic granit­ granitoid rocks has been developed on the ization, the migmatites pass into nebulites, basis of the relationship between the emplace- Geological Survey of Finland, Bulletin 304 39 ment of the plutons and the orogenic folding The synorogenic plutonic rocks are rrrairrly movements. The orogenic plutons, which are gneissose quartz diorites and granodiorites as­ contemporaneous with the folding, are divided sociated with minor bodies of mafic differentiates into two subgroups, synorogenic and late consisting of peridotites, gabbros and quartz orogenic. Both synorogenic and late orogenic gabbros.The plutonic rocks of the granodiorite, bodies were emplaced while the folding was trondhjemite and charnockite provinces are still in progress, and in structure they are in typical representatives of the synorogenic group. harmony with the adjoining country rocks. Synorogenic plutons were emplaced during the In addition, there are some very few small main period of folding. The erogenic defor­ massive postorogenic granitoid plutons, which mation and folding continued after the emplace­ belong, in space and time, to a period of the ment, causing the remarkable gneissose texture Svecokarelidic mountain folding. of the synorogenic plutonic rocks. Gneissose

N N

w EW

0 0-1 % 0 0-1 % W 1 - 2% W 1..-2% []]]] 2- 4% rrm2- 4% S S EEEB4-6 % EEEB4- 6% > 6% N > 6% N - 3 - 4

0

W E W E

0 0-1 % 0 0-1 % W 1- 2% W 1- 2% []]]] 2- 3% mn 2- 4% S S EHE3- 5% EH334- 6% > 5% > 6%

Fig. 25. Structural diagrams of syn()f()9.~tl;icplutoP~~jfg~~~i~gdschists in the Svecofennidic schist-belt of the Tammela-Kalvola ar~a;s(ruthernFi1flarr~~i.$7~iiation,- schistosity, and lineation plotted on the Schmidt net, lower hemisphere: 1, foliation of plutonic rocks (430 samples); 2, schistosity of schists (625 samples); 3, lineation of plutonic rocks (173 samples); 4, lineation of schists (221 samples). After. A.Simonen. 40 Geological Survey of Finland, Bulletin. 304

varieties are especially. abundant at the margins regional metamorphism and deformation took of the plutons, The synorogenic rocks form place in cJlose connection with their emplace­ elongated and ovoidal masses, .concordant with ment. The synorogenic: plutonic bodies were the adjoining country rocks. They have steep emplaced as lenticular masses, in particular side walls, and their structures (foliation and along the bedding and foliation planes in the 'lineation) coincide quite closely with those of anticlines of the folded belt; and their emplace­ the surrounding schists, as is shown in the ment tilted the gentle synclines and. anticlines structural diagrams. of a Svecokarelidic folded into steep isoclinales. area presented in Fig. 25. The emplacement The late orogenic plutonic rocks are granites, of the synorogenic plutonic rocks indicates the which are more massive than the synorogenic climax of the Svecokarelidic orogeny, for the gneissose rock types. The migmatite-forming.

1II

~1

Fig. 26. Stereogram of a dome and sketches showing evolution of migrnatite granite domes. 1, gneisses; 2, migmatite granites. After N, Edelman. Geological. Survey of Finland, Bulletin 304 41 microcline granites are typical representatives of the gently folded belt (Fig. 26, I). Granite of the late orogenic group. In addition, the diapirs rose upward in anticlines. Gneiss syn­ most silicic members of the granite province clines sank between the granite domes and are massive rocks of a late orogenic type. The were compressed into steep. isoclinals. The emplacement of the late orogenic bodies af­ gneisses situated on the tops of the rising fected the adjoining country rocks structurally domes were stretched and flattened (Fig. 26, in many ways, and the resultant ••.structures .are 11).. "Compressionrbetween rthe . rising domes, prominently superimposed on the oldertectonics which were situated one after the other in the of the folded belt produced during the main principal direction of the fold belt, caused period of folding. The structural elements of cross folding (Fig. 26, III).The varying tectonic the late orogenic plutonsand their schist forms may> have been caused by an oblique aureoles deviate from those of the principal cut through the fold belt (Fig. 26, IV). A deeper fold belt. level is .marked by gneiss basins in the granite The late orogenic plutonic rocks occur as area,' whereas an upper level is represented by dome-like bodies or form migmatites with the granite. domes surrounded by continuous gneiss schists and synorogenic plutonic rocks. The belts. A tectonogram, showing old. tectonics of emplacement of the late orogenic granite domes the schist belt deformed by rising domes of is characterized by diapiric updoming, cross migmatitic· granite, is given in Fig. 27. folding, highly plastic migmatite structures, Small stocks of massive granitoids, which and metasomatic granitization. The main phases show. no sign of orogenic movements in their of the structural evolution of the late orogenic structures, .. mark the youngest postorogenic migmatite granite domes in the Svecokarelidic intrusions of the Svecokarelides. The emplace­ fold belt in southwestern Finland are depicted mentof the ·postorogenic bodies. caused the in Fig. 26. Granitization, producing the mig­ gradual bending of the adjoining schists to matite granite, took place in the .lowest .levels follow the contacts of the rounded. stocks. The

5 km ~11

Fig. 27. Tectonogram of the surroundings of Helsinki. Oldsupracrustal tectonics deformed by rising domes of mobilized migmatitic material. 1, metasediments and their relics in migmatite granite; 2, metavolcanics; 3, gneissose granite; 4, rnigmatite; 5, rapakivi-type granite. After E. H. Kranck. 42 Geological Survey of Finland, .Bulletin 304

l====~==;:! 2km N

DEI 1 10d 5 _ 2 /L Ummmml_43 ID 11 ..

:'ta~0J~ ~~ ,;' Angskar

~~~ Angskars fjii.rd

~'

Fig} 28. Geological map of the Ava ring intrusion, Aland Islands, SW Finland. 1, porphyritic granite; 2, irregular dikes of porphyritic granite cutting the migrnatite; 3, monzonite; 4, fine­ grained granite; 5, migmatite; 6, lamprophyre ; 7, diabase dike; 8, quartz porphyry dike.

After S. Kaitaro.. geological map of the Ava granite in south­ been determined for the trondhjemite. Some western Finland, which is associated-with ring granitoids with a zircon age of t-.J 1 900 Ma dikes, is reproduced as 'an example of the yield younger ages ( t-.J 1 800· Ma) for sphene manner of occurrence of the postorogenic and monazite, indicating a late or postorogenic intrusions (Fig. 28). phase of metamorphic recrystallization of the The zircon ages of the Svecokarelidic synoro­ Svecokarelides. The zircon ages of the post­ genic plutonic rocks are usually 1 880 ± 20 orogenic stocks of the Svecokarelides found in Ma. An age of slightly .over 1 900 Ma has different parts of the folded area are t-.J 1 800 Ma.

Postsvecokarelian igneous and sedimentary rocks

The Svecokarelidic orogeny was followed by The great geological events recorded from the a long period of cratonization •and erosion. time ••after. the Svecokarelidic diastrophism were Geological Survey of Finland, Bulletin 304 43 platform-type intrusions of the rapakivi granites, The rap akivi granites (Fig. 29) are pre­ gabbro-anorthosites and diabases of varying dominantly coarse-grained, porphyritic granites ages and the deposition of Jotnian sediments, with large ovoids of orthoclase, many of which are surrounded by plagioclase mantles (wiborgite Rapakivi granites type). In some varieties, there is an almost total lack of plagioclase mantles around the The rapakivi plutons in southern Finland cut potassium feldspar ovoids (pyterlite type). The sharply all the structures of the metamorphic ratio between the mantled and non-mantled and migmatitic rock crust and they were not ovoids varies greatly, and texturally there is influenced by orogenic movements. The plutons a gradual transition from the wiborgite to the are commonly seen to have sharp, outward­ pyterlite. The porphyritic rapakivi granites sloping contacts. Intrusive contacts with sharp­ with porphyritic, angular potash feldspar grains edged, mostly rotated fragments and many are likewise sporadically present and a gradual roof pendants of considerable size commonly transition from this rock to the pyterlite is occur in the rapakivi massifs. observed. The proportions of mantled and non-

Fig. 29. Rapakivi, wiborgite, orthoclase ovoids surrounded by plagioclase mantles. \Tehka­ lahti. 1/3 nat. size. Photo J. Sederholm. 44 Geological Survey of Finland, Bulletin 304

/ 70 ;/ \

80

P 10 30 50 70 90 \N Fig. 30. Ratios between mantled, nonmantled and angular porphyritic potash feldspar grains in the rapakivi rock series, wiborgite-pyterlite-por­ phyritic rapakivi granite, in the Wiborg rapakivi massif. W, orthoclase ovoids mantled by plagioclase (wiborgite type); P, orthoclase ovoids without plagioclase mantle (pyterlite type); A, angular, porphyritic potassium feldspar grains (porphyritic rapakivi granite type). After A. Simonen and A. Vorrna:

mantled and angular porphyritic potassium even-grained rapakivi granite . 7.8 0;0 feldspar grains in the coarse-grained, por­ porphyry aplite . 0.7 0;0 phyritic varieties (wiborgite-pyterlite-porphyr­ quartz porphyry and granite itic rapakivi granites) of the rapakivi in the porphyry . < 0.1 0;0 Wiborg massif are given in the triangular aplite and ...... <0.1 % diagram (Fig. 30). In addition to the foregoing porphyritic The main salic minerals in: all the rapakivi varieties, there are even-grained, granite por­ varieties are potassium feldspar (orthoclase), phyritic, quartz porphyritic, porphyry aplitic, plagioclase () and quartz. Only slight aplitic and pegmatitic types of rapakivi. Further­ deviations occur in their relative contents. more, miarolitic cavities are typical. The areal The most remarkable deviations between the distribution of the different varieties in the different rapakivi varieties appear in the mineral Finnish part of the Wiborg rapakivi massif is associations' of the Fe-Mg-silicates. Hornblende as follows: and biotite are the most common mafic minerals wiborgite . 76.2 % contained in the rapakivi rocks. Some dark- dark-colored wiborgite, . 4.9 % colored, hornblende-bearing varieties contain pyterlite . 6.1 % fayalite and, as alteration products, iddingsite porphyritic rapakivi granite 1.2 % and grunerite, Fluorite, zircon and apatite are dark-colored rapakivi granite . 3.1 0;0 characteristic acces~oryminerals, In some gran- Geological Survey••of Finland, Bulletin 304 45 itic varieties, topaz and are sporadically reservoirs. The diapiric rise of the anatectic present. magma into the upper crust took place after All the rapakivi varieties are chemically erogenic folding in connection with epeirogenic characterized by high contents of silica and movements characterized .by fracturing and potassium, .whereas the contents of Iime :and vertical- block movements. The emplacement magnesia are low. The different rapakivivarieties of ·the rapakivi magma took place into the show only slight variation in their chemical deeply denuded. roots of the cratonized, Sveco-­ composition. In the Wiborg massif, the dark... karelidic·· folded belt.: The rapakivi massifs, colored wiborgite and dark-colored even-grained connected with. effusive counterparts, represent granite contain less silica and more FeO,MgO typical disharmonious or epizonal granites. and CaO·· than •the other rapakivi varieties. The many rapakivi intrusions are closely­ Furthermore, the contents of FeO, MgO and associated with mafic magmatites .(basaltic lava CaO in. wiborgite are higher than those in flows.. ·diabases, anorthosites), which indicate pyterlite. Pyterlite, porphyritic rapakivi granite, the deep-seated origin of. the magmas. It is­ even-grained rapakivi granite, porphyry aplite probable, that both juvenile melts and anatectic and . quartz .porphyry are chemically closely magmas t?ok part in the lively Postsvecokarelian related. Small differences in chemical compo­ magmatism in southern Finland. sition between rapakivi rocks of different massifs The age· deterrninations by the U-Pb method have also been observed. on zircons. show that the crystallization of the The best evidence of the magmatic origin rapakivi granites occurred 1700-1 540 Ma ago. of the rapakivi is proved by the sharp intrusive The •ages. of the different massifs vary and, contacts and effusive types (quartz porphyries furthermore, in some massifs there are many and granite porphyries) associated with the magmatic •.•phases.:of different ages. The oldest rapakivi granites. The emplacement of the rapa­ phases .of the rapakivi granites have been re-­ kivi magma produced thermometamorphic corded from theWiborg and Aland massifs, aureoles around the massifs. Mineral assem­ the ages. of which are 1 700-1 640 Ma and blages of the pyroxene- facies •have 1·670 ± 20 Ma, respectively. Three major­ been found in the and country rocks magmatic. phases, 1 700-1 660 Ma, 1 660­ along the innermost contact zones of the rapa­ 1 640 Ma and 1 640 ±.15 Ma, have been kivi massifs. Furthermore, the original micro­ tentatively reported from the Wiborg massif. cline of the country rocks (primarily meta­ The .most .probable ages of the Vehmaa and morphosed under the low-pressure conditions 'massifs are 1 590· Ma and' 1 570 Ma,. of amphibolite facies) was .transformed around but the youngest intrusive phase in these the rapakivi massifs into orthoclase owing to massifs occurred only 1 530-1 540 Ma ago. heat generated by rapakivi. The orthoclase aureole around the big massif may be up to Aiaj7c igneous.rocks 5 kilometers broad, but around small massifs it is much narrower. The emplacement tempera­ A· cornmon • occurrence was the intrusion ture; of the rapakivi magma has been estimated ofmafic· igneous rocks, mainly diabases and to exceed 800 "C. gabbro-anorthosites, into the craton stabilized The rapakivi magmas may have originated after the Svecokarelidic orogeny. These mafic by ultrametamorphism during the culmination rocks are associated with either rapakivi of the Svecokarelidicorogeny in of the Jotnian sediments.. crust and remained in a liquid state are only slightly older than periods of time, capsuled in their rocks, whereas' some of them 46 Geological Survey of •Finland, Bulletin. 304 are younger than the Jotnian sediments, Small ancient surface and. produced subvolcanic and masses of anorthositic gabbros and anorthosites volcanic rocks. At the southern margin of the occur. in connection within many rapakivi Wiborg .rapakivi massif, on the island of Hog­ plutons. The largest is . the horseshoe-shaped land in the Gulf of. Finland, composite lava gabbro-anorthosite complex around the Ahve­ flows, consisting .of porphyrites and quartz nisto rapakivi massif. The gabbro-anorthosites porphyries comagmatic with the rapakivi, overlie are coarse-grained and their main minerals the Svecokarelidic rocks. Further, the study of are plagioclase (An4 8 -58), • orthopyroxene, am­ roof pendants in the \X7iborg rapakivi massif phiboles and biotite. SmalL.bodies of anortho­ has shown that the roof consisted partly of sites are present, composed of plagioclase Svecokarelidic rocks i and partly of Postsveco­

(An 5 3 - 5 8) with a mafic mineral-content of less karelian mafic and silicic volcanic rocks.' The than 10 per cent. Furthermore, in the middle mafic volcanics are' diabases and lava flows of of the gabbro-anorthosite intrusions there occur plagioclase porphyrites and they are older than pegmatoidic bodies consisting mainly of pla~io­silicic porphyries interpreted partly as ignimbrite clase and hypersthene crystals. up to one meter eruptions. The porphyries caused hybridization in length. The age of the gabbro-anorthosite of the diabase, and the rocks of the roof pendants around the Ahvenisto rapakivi massif is were metamorphosed thermally by the rapa­ t-' 1 680 Ma, or only slightly older than the kivi granite. The' zircon age of the diabase in rapakivi, which penetrates the mafic rocks. the roof pendant; is ~I1 690 Ma and that of In southern Finland. there occurs a diabase the porphyry t-' 1 685. Ma, dike set about 150 km .Iong, the main trend The foregoing mafic igneous rocks (gabbro­ of the vertical or steeply dipping dikes is anorthosites, diabases and lava flows) mark the r N 60 ow• The easternmost continuation of the emplacement of the basaltic 'magma into dif­ dike set is situated in southeastern Finland on the ferent levels of the crust; The field relation­ northern border of the rapakivi massifs, and ships and age data indicate that these mafic the diabases are penetrated .by rapakivi. ,The igneous rocks are only slightly older than the deep faults and the opening of, the fractures oldest intrusions and extrusions of the rapa­ controlling the diabase intrusions were prob­ kivi lnagmla. The close association of the mafic ably caused by the upward movement of the igneous rocks and rapakivi granites has been rapakivi magma. The diabases cut themeta­ pointed out by many Nordic. geologists, and morphic and migmatitic rocks, and they show these rocks have been usually named »Sub­ chilled contacts against the wall rocks. Their jotnian» in the age classification of the Pre­ main minerals are plagioclase, clinopyroxene cambrian in Fennoscandia. and olivine, ~anyof' the diabase occurrences Diabase dikes and sills younger than the contain autoliths and plagioclase megacrysts. Jotnian sediments occur abundantly in as­

Moreover, quartzite xenoliths are characteristics sociation 'with the ••Jotniansandstone area of of many of the diabase dikes. Rheomorphic Satakunta and in the archipelago of south­ dikes, produced by the heat of intruded diabase western and western Finland. They show both magma from different rocks of the metamorphic vertical' and horizontal. chilled contacts against complex, are common. The diabase dikes are country rocks consisting of the Svecofennidic only slightly older than. the oldest intrusions rocks, rapakivi granites and Jotnian sand­ of the rapakivi magmas. stones.: Diabases represent. hypabyssal eruption It is noteworthy that. the intrusions of the channels of basaltic magma intrusions into a magma, which formed the foregoing gabbro­ stable platform characterized by faulting tec­ anorthosites and diabases, also reached the tonics. Geological Survey of Finland, Bulletin 304 47

The main minerals of these Postjotnian dia­ characteristics of the are typical of bases are plagioclase, augite and olivine, The the red, immature sandstones of the arkose chemical composition is basaltic, similar to suite. The feldspar is predominantly potassium that of plateau basalts. feldspar and it occupies 20-40 per cent of the Contact phenomena of the diabase, resulting total amount of sand particles, whose roundness in partial or almost complete melting of country and sorting. are poor. The stratification of the rocks, are common. Partial melting of various sandstone is generally horizontal and many :silicic country rocks, located in the immediate sedimentary structures indicating terrestrial con­ vicinity of the diabase contact, produced an ditions .of. deposition can be observed. Current intergranular melt of eutectoid granite com­ bedding is common and ripple marks, made position. This melt intruded from the host by water, are symmetric or asymmetric. rock into the chilled contact zones of the diabase Furthermore, mud cracks, clay galls, and rain and resulted in the formation of »palingenic drop impressions have been found. dikes», consisting mainly of quartz and alkali The J otnian sediments in the Muhos area feldspar. The reactions between the anatectic begin with basal conglomerates and arkoses granite melt and the solidified diabase caused lying unconformably upon the metamorphic .hybridization of the original contact zones. basement complex. The conglomerates contain So far is known, the Postjotnian diabases rounded and angular pebbles of granite and in southwestern Finland represent the youngest schist and the is brown-colored arkose Precambrian rocks in the country. These dia­ rich in silty material. The red arkosic sandstone bases give zircon and Rb-Sr ages ranging be­ of Muhos is similar to the Satakunta arkose. tween the limits 1 250-1 275 Ma. The main part of the Muhos sediments, many hundred meters thick, consists of red, brown Jotnian sediments or greyish green and shales with thin interbcds of arkosic sandstone. The red and A long period of denudation followed the brown siltsones and shales are predominant, in trusions of the rapakivi rocks, and the J otnian forming 80-90 per cent of the total thickness sedimentswere deposited on the deeply eroded of the sequence. craton, especially in the grabens and depressions The general characteristics and the red color caused by vertical block movements. So far of the afore-mentioned J otnian sediments in­ as is known, the J otnian sediments represent dicate an oxidizing condition of terrestrial the oldest unmetamorphic sedimentary cover sedimentation. The J otnian deposits in the of the Baltic Shield. Satakunta and Muhos areas are related to The Jotnian deposits of Finland occur in floodplain deposits in the piedmont facies of downfaulted blocks of the folded Precambrian postgeosynclinal basins. complex in Satakunta and Muhos. The drillings The J otnian shales were dated by I<'-Ar show that the Muhos graben has been down­ and Rb-Sr methods. The tentative age of the faulted about one kilometer and the Satakunta rocks is 1 300-1 400 Ma. Mention must be graben at least 650 meters. It is further .note­ made of the fact that the j.otnian arkoses of worthy that J otnian sediments occur abundantly Satakunta are penetrated by the diabases, at the bottom of the Gulf of Bothnia. whose age is ~ 1 270 Ma. The J otnian sediments of. Satakunta consist of red-colored, stratified arkosic sandstQij~§ with thin intercalated beds of red or black the deposition of the J otnian shales. The mineralogical as well as chemical sediments produced a peneplain upon which 48 Geological Survey of Finland.iBulletin 304 the Cambrian transgression took place. Sporadic Carnbrian 'sandstone, spilled as. sand' from the occurrences of Cambrian quartz sandstone and surface to the fissure cracks and solution cavities> other Cambro-Silurian sediments have been indicate ,that the.' ancient .'sub-Cambrian pene­ found in .southwestern Finland and, along the plain, on which these sandstones were deposited,. rand zone of the Caledonidic mountain chain has been situated. slightly above the present in the northwesternmost part of Finnish ·Lap­ surface. Therefore;' the •erosion of ,.the ''Pre-· land. However, most, of these sediments were cambrian. crust' after Cambrian time was very later removed by· denudation. The Cambrian slight.. The sub-Cambrian peneplain is still seen sandstone in' southwestern Finland occurs com­ as a tangential upper surface of the hills and monly as' sandstone dikes in •Precambrian. rocks this peneplained Precambrian surface dips gently or as fillings of solution cavities in Precambrian under the sedimentary cover ••of. the Russian crystalline , limestone. '.••These < remains ••of the platform.

CONCLUDING ·REMARI

The most important', phases of the evolution karelidic schist,.zones occur only as narrow, .of the Finnish Precambrian are .summarized steep ·isoclinales between the •blocks of base­ in the geological timetable presented in Fig. 31. ment gneisses and they contain a profusion of Many geological events of different ages have mafic and. ultramatic metavolcanics; Abundant been. documented in the timetable" which '.also igneous activity. in the earliest geological times, shows long periods Jacking documents on the may indicate that .magnla ' masses ••had existed ancient history of the Earth. at rather shallow depths beneath the crust, The most important events in the geological The metasediments are mainly.micaceous schists,. evolution of Finland have been the 'revolutions and graphite-bearing black schists occur as· producing the Presvecokarelidic and Sveco­ interbeds. Sedimentary limestones. are. lacking. karelidic folded areas. These revolutions ·took Quartz:-bandediron formations are characteristic place 2 800-2 600 Ma and 1 900---1800 .'Ma and suggest.· an •anoxygenous environment of ago, respectively, and most of the Finnish ancientatmospherer-No valid evidence of the Precambrian rocks received'. their present. ap­ existence of relics of ancient organisms has. pearance during these ancient events. The great been 'known. The' characteristics of, the Pre­ revolutions connected •with folding, plutonism svecokarelidic schists indicate conspicuous simi­ and metamorphism lasted. a relatively short larities to so-called Archean greenstone belts time in the long Precambrian history. The time of the other . during which the endogenic processes had The folded basement area •stabilized about been in progress was suprisingly short and the 2 600·' Ma ago and the denudation started. An rocks produced have slept,.a long •time, like igneous activity in this stabilized craton took Sleeping Beauty. place about 2 400 Ma ago, and it is represented The oldest Presvecokarelidic folded area mainly by Presvecokarelian mafic intrusions, (2 800-2 600 Ma) of the Finnish Precambrian which form layered sheet- or lopolith-like bodies. is characterized by broad areas of granodioritic The' emplacement. of the carbonatite complex basement gneisses in eastern and northern of Siilinjarvi, which is ••one of the oldest car­

Finland. True potassium-rich granites, which bonatite .•.occurrence in the world, also took are usually common in .thc younger. folded place ·into the. stabilized 'basement 2400-2 500 areas" do not occur abundantly.• The Presveco- Ma ago. Geological Survey ·0£ Finland, Bulletin 304 49-

I Time Ma GEOLOGICAL···E\lOLUTION·.·OFF1NLAND PLEISTOCENEGLACIATI

ALKALINE ROCKS AND CARBONATITES CALEDONIDES 500 PALEOZOIC·SEDIMENTS

:1000

POSTJOTNIANDIABASES JOTNIANSEDIMENTS

1500 RAPAKIVI GRANITES POSTSVECOKAREUAN IGNEOUSROCKS{QUARTZ PORPHYRIE;S { . DIABASES .

SVECOKARELIDES FOLDING AND PLUTONISM GEOSYNCLINAL VOLCANISr~ GEOSYNCLINAL SEDIMENTATION 2000 ( KA LEVIA N AND.•SVECOFENNIAN) • JATULlAN VOLCANISM · { AND SEDIMENTATION

PRESVECOKAR.ELIANIGNEOUS ROCKS {LAYERED MAFIC INTRUSIONS 2500 (YOUNGERTHANTHEBASEMENTCOMPLEX).•CARBONATITECOMPLEX

GRANULITES PRESVECOKARELIDIC BASEMENT GNEISSES BASEMENT { SCHISTS

Fig. 31. Geof5gical timetable of the Precambrian in Finland. Compiled by A.. Simonen.

The Presvecokarelidic basement .was .deeply the of the Jatulian group (basal eroded and peneplained about 2 300 dolomites and sapropelic pe­ and forming the 'so-called Jatulian continent. lites) some 2 300-2200 Ma ago.· The

Upon this continent and along ..its borders, epicontinental, transgressive group of Jatulian 50 Geological Survey of. Finland, Bulletin 304 sediments is separated by a marked uncon­ of the Svecokarelides marking the main re­ forrnity from thePresvecokarelidicbasement volutionary phase of the orogenic movements. and it is overlain by thick accumulations of The main phases of the evolution of the Sveco­ Kalevian phyllites and mica schists, which are fennides are summarized in Table 2. products of incomplete chemical weathering The Karelian and Svecofennian rocks, which and show characteristics of geosynclinal sedi­ accuma1ated upon the Presvecokarelidic base­ ments. ment and in the geo:synclina1 basins, meta­ The time of the Jatulian sedimentation was morphosed into crystalline schists during the associated with lively igneous activity, rep.­ Svecokarelidicfolding, 'which took place about resented by mafic volcanics and diabases oc­ 1 900 Ma ago. In close connection with the curring as interbeds, sills and dikes in the orogenic movements, enormous masses of Jatulian sequence. It is noteworthy that the granitoids (1 900-1800 Ma) emplaced into the Jatulian diabases penetrate both the Presveco­ schists, The duration of the whole Svecokarelidic karelidic basement and the Jatulian sediments, orogenic cycle (from evolutionary sedimentation but not the Kalevian rocks overlying the Jatulian until the intrusion of the postorogenic granites) group. is about 500 million years in the interval ranging The pulses of the Jatulian magmatism range from 2 300 Ma to 1 800 Ma; but the duration between the limits of 2 200-2 000 Ma, dated of, the revolutionary phase, connected with by the ·U-Pb method on zircons. The Pb-Pb folding, metamorphism and intrusions of oro­ age of the dolomite and the age (total lead of genic plutonic rocks, is only about 100 million the whole rock, isochron) of the iron formation, years (1 900-1 800 Ma). Mention must be both rocks belonging to the upper part of the made of the fact that certain parts of the Pre­ Jatulian group, are ~ 2 050 Ma and ~ 2 080 svecokarelidic basement (for example: the cores Ma, respectively. On the basis of the ages of the mantled gneiss domes and the granulite mentioned, it is possible to conclude that the belt) participated in the Svecokarelidic move­ time of the .Jatulian sedimentation. continued ments, becoming rejuvenated and remobilized. over 200 Ma (~ 2 200-2 000 Ma ago). The It can be stated that the Finnish Precambrian Kalevian sediments, overlying the J atulian, 'are gained its present appearance for the greatest younger than ~ 2 050 Ma, but older than the part within the Svecokarelidic orogeny. synorogenic Svecokarelidic plutonic rocks The metasediments.and metavo1canics of the (~ 1 900 Ma) which penetrate them. Svecokarelides are in tnany areas quite well­ The geosynclinal basins developed to the preserved and show, along with relics of primary west of the /ancient Presvecokarelidic craton structures and textures, rnany actualistic features. and thick accumulations of Svecofennian geo­ The fact should be noted that the research synclinal deposits took place. The immature done on these rocks at the end of the 19th sedimentsare mainly graywackes and arkoses. century proved, for the first time, the actualistic This sedimentation is closely associated with method to be the correct approach in the study geosynclinal volcanisrn, showing eugeo­ of metamorphic Precarnbrian formations.. Fur­ synclinal type of deposition. The Svecofennian thermore, the deposited rock sequences and geosyncHnal sedimentation and volcanism took place immediately before and during the climax their folded structures are commonly related of the Svecokarelidic orogenic movements to those of the younger. orogenic belts. Owing about 1 900 Ma ago. The age of the Sveco­ to the deep erosion of a section of the Sveco­ fennian volcanism is 1 920-1 880 Ma and only karelides, the abundance of orogenic plutonic slightly older than the synorogenic plutonism rocks is a special feature, indicating that the Geological Survey of Finland, Bulletin. 304 51

Table 2. Precambrian evolution of the Svecofennidic terrain in Finland. After A. Simonen.

Phases Processes Rocks

Time of cratonization: Postjotnian . intrusion diabase dikes Jotnian . sedimentation sandstones •and siltstones Subjotnian . intrusion rapakivi granites and diabase

SVECOFENNIDES:

Orogenic phases: Postorogenic phase ...... •...... intrusion postorogenic granitoids Late-orogenic phase . emplacement of late-orogenicgran­ migmatitesand migrnatite-forming ites, migmatization, granitization, granites diapiric updoming and cross folding Intraorogenic phase . jointing and intrusion amphibolite dikes Synorogenic phase . emplacement •of synorogenic plu­ synorogenic plutonic rocks (mainly tonic rocks, regional' metamor­ quartz diorites and granodiorites), phism, deformation and folding metamorphic schists (mainly mi­ caceous schists and gneisses, quartz­ feldspar schists and amphibolites) Geosynclinal phases of deposition: Upper. Svecofennian . sedimentation argillaceous sediments Middle Svecofennian . volcanism and sedimentation mafic volcanics and intercalated sediments Lower Svecofennian . sedimentation mainly graywackes . and impure arkoses

Presvecofennidic . basement unknown

root zone of the folded belt was invaded by intrusions and the emplacement of gabbro­ ernormous masses of granitoid rocks. anorthosites, diabases and volcanic rocks (por­ The most valid signs of ancient life (Figs. phyrites and quartz porphyries), connected 32-33) during the sedimentation of Sveco­ closely with extremely lively igneous activity karelian strata are stromatolite structures in of the rapakivi, took place in the stable plat­ the Jatulian dolomites and carbonaceous sacs form 1 700-1 550 Ma ago. The intrusions of ( Corycium} in the Svecofennian graywacke­ rapakivi and related granites seem to be a slates of the Tampere field. Furthermore the special feature of the crustal evolution in the black schists occurring as interbeds have been Middle Precambrian of many continents. interpreted to be bituminous, sapropelic sedi­ The Jotnian red beds, representing oxidized ments. terrestrial sediments, were deposited about The evidence relating to the geological 1 400-1 300 Ma ago upon the deeply eroded evolution of the Finnish Precambrian after the Precambrian surface. They form the oldest Svecokarelidic orogeny is highly sporadic and sedimentary cover of. the metamorphic Pre­ full of gaps. The Svecokarelidic orogeny\vas calubjIial1.Crust. The Postjotnian diabases (~ followed by a long period of erosion and 1 275 Ma) are the youngest Precambrian igneous cratonization. The many pulses of. rapakivi rocks in Finland. No Precambrian sedimentary 52 Geological Survey of Finland, Bulletin 304

Fig. 32. Stromatolite structure in dolomite. Ollitervo, Tervola. After M. Harrne and V. Perttunen.

o 5 I I

Fig. 33. Carbonaceous sacs (Corycium) in graywacke-slate, Tahtinen, Aitolahti, Tampere. Photo E. Halme.

rocks younger than the Jotnian have been of Finland was denuded almost to its present found, but the erosion continued and at the level. dawn of Cambrian time the Precambrian crust

ECONOMIC GEOLOGl:

The Precambrian rock crust of Finland con­ rise to mining operations. The most important tains many different types of deposits, in­ mines and quarries operated at present are dustrial minerals and rocks, which have given shown in Fig. 34. Geological Survey of Finland, Bulletin 304 53

RAUTUVAARA 114ft Fe .1 AKASJOENSUU .3"2

VIHANTI .Zn,Cu,Pb OTANMAKI fi 0LAHNASLAMPI V,Fe,Ti0 2

HITURA • Ni,Cu .PYHASALMI Cu,Zn,S RYYTIMAA KINAHMI HIEKKAMAKI ~ SIILlNJARVI ~ POLVIJARVI/ LUIKONLAHTI • ,pVUONOS CU,Zn,Co,S ~ CU,Zn,Ni,Co • KERETTI. J K~TALAHTICZ COS HAMMASLAHTI NI,Cu U, n, Cu ANKELE ~.VIRTASALMI. I VIRTASALMI<@> RUOKOJARVI Cu

• VAMMALA Ni,Cu

IHALAINEN~

~ PARAINEN~~ TYTYRI o 50 100 150 ~~~~SIPOOI : ! r : KEMIO KM FORBY~MUSTIO

Fig. 34. Mines and quarries in ore; 3, chromium ore; 4,. limestone; quartz. S4 Geological Survey. of Finland, Bulletin 304

Ore deposits ores of the Belt are divided .into the Outokumpu ore dis trict and the ore zones of. Vihanti and deposits are The ore now mined in Finland I

1 Kemi ., . Cr 800000 (Kalevian) micaceous schists. The serpentinite 2 Mustavaara . V 1 750 000 lenses of the Outokunlpuzone are in many 3 Otanmaki . V, Fe, Ti0 2 1 250 000 4 Rautuvaara . Fe 1 500000 places rimmed by dolomite, skarn and quartzite. 5 Vihanti . Zn, Cu, Pb 800000 6 Pyhasalmi . Cu, Zn 1 000000 The dolomite 'may bea product of metasornatism 7 Hitura . Ni, Cu 400000 of the serpentinite, and the quartzite may have 8 Vuonos 1Outo- Cu, Zn, Co, S 400000 9 Keretti f kumpu Cu, Zn, Co, S 400000 originally been a sinter-Iike, chemical silica 10 Kotalahti . Ni, Cu 500000 precipitate. The skarn with green chrome­ 11 Luikonlahti . Cu, Zn, Co, S 350 000 12 Harnmaslahti . Cu 400000 bearing minerals (diopside, tremolite, uvarovite, 13 Virtasalmi . Cu 300000 etc.) is a reaction product between carbonate 14 Varnmala . Ni, Cu 300000 and. silicate rocks. 'The ores are situated in quartzitic rocks, near the contact of serpentinite. Most of the deposits are sulphide ores. Some The ores were probably. produced by the early of them are -copper deposits associated geosynclinal magmatisrn represented by the with mafic and ultramafic plutonic rocks of ultramafics of the Outokumpu zone. The lead the Svecokarelides and· some sulphide ores model age of the galena in the Outokumpu containing copper, zinc, cobolt, nickel and lead is ~ 2100 Ma. and situated in the Svecokarelidic schist belt. The ores of the Vihanti zone (Vihanti and Among the oxide ores. are. the deposit Pyhasalmi in Table 3) arepolymetallic sulphide of Kemi and iron ore occurrences containing deposits containing •zinc, copper and lead as in some cases vanadium. and titanium. well as a little and silver. These deposits The most important sulphide occurrences are situated in Svecokarelidic schists composed (over 90 per cent of the known reserves) are of metasediments and metavolcanics of varying situated in the »Main Sulphide Ore Belt», composition. An abundance of skarns and which runs diagonally across the .country from cordierite-anthophyllite rocks in the sur­ to the northern coast .of the Gulf roundings of the ore deposits is characteristic. of Bothnia (Fig. 35). This Belt runs roughly The ores seem to be associated with the acid in the direction of the boundary between two volcanism of the region. The lead model ages main structural units of the Finnish Precambrian, of the galenas from the Vihanti zone are 1 925­ the Presvecokarelidic craton and the Sveco­ 1 975 Ma. karelidic folded area. The southwestern side The ores of the Kotalahti zone (Hitura and of the Belt is characterized by a deep negative Kotalahti in Table 3)· are sulphide deposits gravimetric anomaly, and many fault lines, containing nickel and copper. The host rock which have been active in different times, are of the ores is serpentinite, pyroxenite or norite. located in the direction of the Belt. The sulphide The ore deposits are early magmatic and as- Geological Survey of Finland, •Bulletin 304 55

/ • 7 2 c 8 3 9 9 IZl 4 , 10 ~ihanti [Z] 5

IIIIIIII11111 D 6 50 ~ 1 Hitura IIIII~IIIIII% 01:::==1 :=:t::=:===:::I,qOkm Makola ~111111~hasalmi

I~II •1IIIIIItUikonlahtiIII 111// lEuonos 1 Kotalahti II~I[outoK~;1,

1II1III11 Hammqs- III Ichti

1111111111 Virtasalm i' 11/

~IIIIIIIIIIIIIII

29°

Fig. 35. Main Sulphide Ore Belt. 1, Kotalahti zone (Ni, Cu); 2, Vihanti zone (Zn, Cu, Pb); 3, Outokumpu district (Cu, Co, Zn, Ni); 4, serpentinites of the Outokumpu zone; 5, Pre­ svecokarelidic basement; 6, Svecokarelides; 7, Ni-mine; 8, Zn-Cu-Pb mine; 9, Cu-Co mine; 10, Cu-mine, After A. Kahma.

sociated with .the Svecokarelidic plutonism plutonic rocks, but this ore body is situated (~ 1 900 Ma). far from the Main Sulphide Belt. Other sulphide deposits, deviating from the The oxide ore mines (Kemi, Mustavaara, afore-mentioned ore types or occurring outside Otanmaki and Rautuvaara in Table 3) are the Main Sulphide Belt, are being mined at located in northern Finland. The oxide ores, Hammaslahti, Virtasalmi and Vammala. The with the exception of the Rautuvaara deposit, ores of Hammaslahti and Virtasalmi are situated are genetically associated with mafic and ultra­ in the Svecokarelidic schists and they are copper mafic plutonic rocks. The chromite ore of deposits with no economically significant Kemi and the iron-vanadium ore of Musta­ amounts of other metals. The epigenetic origin vaara are associated with the differentiated, of the Hammaslahti ore has been pointed out: layered mafic intrusions of the age group material was remobilized from the host rocks. ~ 2 400 Ma, whereas the titaniferous iron ore The copper ore of Virtasalmi has been ex­ of Otanmaki seems to have been associated plained as a contact-pneumatolytic deposit. The with the initial Jatulian magmatism of the nickel-copper deposit of Vammala is associated Svecokarelides. The magnetite iron ore of with the Svecokarelidic mafic and ultramafic Rautuvaara is situated in the Svecokarelidic 56 Geological Survey of Finland, Bulletin 304 schist zone, and its surroundings are charac­ Granite and slate as well as rocks used in terized by skarns,A metasornatic origin of the the production of cement and mineral wool are Rautuvaara ore has been pointed out. quarried in many places throughout the country, mostly on a small scale. The even-grained, Industria! minerals and rocks homogeneous red granite of Vehmaa, in south­ western Finland, is one of the most frequently In addition to the afore-mentioned ore de­ used granites in Finland. The Vehmaa granite" posits some occurrences of industrial minerals commercially known as »Balmoral red», belongs and rocks are mined and quarried. The most to the group of rapakivi granites. Grey granite important are the limestone mines and quarries has been quarried in, for example, the archi­ (both calcitic and dolomitic) .in the different pelagoof (trondhjemite) and in parts of the Svecokarelidic area (see Fig. 34). the district of Kuru (north of the town Tarn­ The carbonatite of the Siilinjarvi complex (cf., pere). Black granite (diorite, gabbro) has been. p. 23) is quarried for apatite, and the apatite­ quarried in the districts of Hyvinkaa, Kuru and bearing Paleozoic carbonatite ..stock of Sokli Jyvaskyla, for example. The granites of different in northern Finland is under investigation. The colors are quarried mostly to produce monu-­ serpentinitcs of the Karelidic belt have been ments, tombstones and stone for building' altered by metasomatism into talc-bearing rocks, facades. Some rocks, usually with exceptional which are quarried for talc in Lahnaslampi color, are quarried for ornamental purposes and and Polvijarvi, A nickel concentrate (pent­ some minerals (chrome. diopside, smoke quartz,. landite) is separated as a by-product of the talc jasper, garnet, labradorite with labradorescense, industry. etc.) are used as gemstones. As a curiosity, it Many bodies of complex granite pegmatites with rare minerals occur. in different parts of might be mentioned that alluvial gold occurs in. the country. Only tVTO of these, Kernio and Lapland along the southern part of the granulite Haapaluoma, are quarried at present for feldspar arch and some private gold-diggers as well as, and quartz. Sericite quartzite is quarried in some amateurs and tourists dig and pan .for Kinahmi and Hiekkamaki, also for quartz. the precious metal during the short summer.

ACKNOWLEDG MENTS

I wish to thank my colleagues in Finland and neigh­ Finland, and Professor Toivo Siikarla, head of the boring .countries for many helpful discussions con­ Geophysical Department of the Geological Survey, cerning the geological problems of the Baltic Shield. kindly advised me on the geophysical data. Mr. Paul Dr. Olavi Kouvo made available to me the age deter­ Sjoblorn, 'M" A., corrected the English text. To all the minations carried out under his leadership in the Geo­ persons who have assisted me and given me inspiration, chronological Laboratory of the Geological Survey of I want to express my sincere thanks. SELECTED PAPERS ON THE PRECAMBRIAN IN FINLAND

Eskola, Pentti: The Precambrian of Finland. Pp. 145­ Korsman, K.: Progressive metamorphism. of the meta­ 263 in The Precambrian. V01. 1, ed. by K. Rankama. pelites in the Rantasalmi-Sulkava area, southeastern Interscience Publ., New York-London-Sydney, Finland. Geol. Surv. Finland, Bull. 290, 1977. 82p. 1963. Kouvo, 0.: Kallioperamme kronostratigrafiasta. Strati­ Kahma, Aarno: The main metallogenic features of grafiasymposio 8. 9. 1976. Suomen Geologinen Seura Finland. Geol. Surv. Finland, Bull. 265, 1973. 28 p. ja Geologiliitto ry. Koulutusmoniste Tsl.o 2, 1-13. Sederholm, J. J.: Pro-Quaternary rocks of Finland. 1976. Bull. Comm. geol, Finlande 91, 1930. 47 p. Kouvo, O. & Tilton, G. R.: Mineral ages from the - On the geology of Fennoscandia with special refe­ Finnish Precambrian. J. Geol., 74, 421-442. 1966. rence to the pre-Carnbrian. Bull. Comm. geol. Laajoki, K.: On the geology of the South Puolanka Finlande 98, 1932. 30 p. area, Finland. Geol. Surv. Finland. Bull. 263, 1973. Simonen, Ahti: Pre-Quaternary rocks in Finland. Bull. 54 p. Cornm. geol. Finlande 191, 1960. 49 p. Laajoki, K. & Sakko, M.: '\X/hole rock Pb-Pb ischron - Kalliopera. Pp. 49-124 in »Suornen geologia», ed. age for the Paakko iron formation in Vayrylankyla, by K. Rankama. Kirjayhtyrna, Helsinki, 1964. South Puolanka area, Finland. Bull. geol. Soc. Fin­ - Das finnische Grundgebirge. Geol. Rundscha~60, land 47, 113-116. 1975. 1406-1421, 1971. Laitakari, I.: On the set of olivine diabase dikes in Vdyrynen, Heikki: Suomen kalliopera, sen synty ja Hame, Finland. Bull. Comm. geol. Finlande 241, geologinen kehitys. Otava, Helsinki, 1954. 260 p. 1969. 65 p. Laitala, M.: On the Precambrian bedrock and its struc­ New papers: and special problems ture in the Pellinge region, South Finland. Geol. Surv, Finland, Bull. 264, 1973. 76 p. Gravity tectonics folding a basic Ehlers, c.: and around , E.: Evolution of the Precambrian volcanic SW' volcanic centre in the Kumlinge area, Finland. complex in the Kiuruvesi area, Finland. Geol. Surv. Geol. Surv. Finland, Bull. 295, 1978. 43 p. Finland, Bull. 283, 1976. 109 p. Gaal,. G. & Rauharnaki, E.: Petrological and structural Matisto, A.: Corycium enigmaticum. Bull. Comm. analysis of the Haukivesi area between Varkaus and geol, Finlande 268, 1974. 30 p. Savonlinna, Finland. Bull. Geol. Soc. Finland 43, Merifairien, K.: The granulite complex and adjacent 265-337. 1971. rocks in Lapland, northern Finland. Geol. Surv. Huhma, A. & Huhma, M.: Contribution to the geology Finland, Bull. 281, 1976. 129 p. and of the Outokumpu region. Bull. N uutilainen, On the geology of the Misi iron ore Geol. Soc. Finland 42, 57-88. 1970. "C"~~"~ J.: province, Finland. Ann. Acad. Scient. Hyvarinen, L.: On the geology of the copper ore field northern in the Virtasalmi area, eastern Finland. Bull. Com~ Fennicae, Ill, Geol.-Geogr. 96, 1968. 98 p. geol. Finlande 240, 1969. 82 p. ,'_._~~~~Nykdnen, 0.: On the Karelides in the Tohmajarvi Harme, M.: On the potassium migmatites of southern area, eastern Finland. Bull. Geol. Soc. Finland 43 Finland. Bull. Comm. geol. Finlande 219, 1965. 43p: (2), 93-108. 1971. Harme, M. & Perttunen, V.: Stromatolite structures Paakkola, J.: The volcanic complex and associated in Precarnbrian dolomite in Tervola, North Finland. manganiferous iron formation of the Porkonen­ C.R. Soc. geol. Finlande 35, 79-82. 1964. Pahtavaara area in Finnish Lapland. Bull. Comm, Juopperi, A.: The magnetite gabbro and related Musta­ geol. Finlande 247, 1971. 83 p. vaara vanadium ore deposit in the Porttivaara layered , Peltola, E.: Origin of Precambrian copper sulfides of intrusion, northeastern Finland. Geol. Surv. Finland, the Outokumpu district, Finland. Econ. Geol. 73 Bull. 288, 1977. 68 p. (4), 461-477. 1978. 58 Geological Survey of Finland, Bulletin 304

PenttiHi, E.: Crustal structure in Fennoscandia from Simonen, A.: Evolution. of the Svecofennides in Fin­ seismological and gravimetric observations. Ann. Acad. land. 22th Internat. Geol. Congr., India 1964, Section Scient. Fennicae, Ill, Geol.-Geogr. 110, 1972. 38 p. 10, 198-209. 1964. Piirainen, T.: Die Petrologie und Uranlagerstatten des - Batholiths and their otogenic setting. American Koli-Kaltimo Gebietes im Finnischen N ordkarelien. Geophysical Union, Geophysical Monograph 13,483__

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Piirainen, T. & Hugg, It. &Isohanni, M. & JuoP­ Simonen, A.. & Vorma, ..A.•: Amphibole and biotite peri, A..: On the geotectonics and ore forming pro­ from •rapakivi. Bull. Comm. geol. Finlande 238, 1969., cesses in the basic intrusive belts of Kemi-Suhanko 28 p. and Syote-s-Narankavaara, northern Finland. Bull. Simonen, All & Vorma, A.: The Precambrian of Fin-­ Geol. Soc. Finland 46, 93-104. 1974. land. Metamorphic map of Europe. 1: 2 500 000., Piispanen, R.: On the spilitic rocks of the Karelidic Explanatory text. Subcom:mission for the Cartography belt in western Kuusamo, northeastern Finland. of the Metamorphic Belts cf the \XlorId. Leiden­ Acta Univ.. Ouluensis.· Geologica 2, 1972. 73 p. Unesco, Paris. 1978. 20--27. Puustinen, K.: Geology of the Siilinjarvi carbonatite Vaasjoki, M.: Rapakivi granites and other postorogenic complex, eastern Finland. Bull. Cornm. geol, Finlande rocks in Finland: .their age and the lead isotopic 249, 1971. 43 p. composition of certain associated galena mineral­ Rouhunkoski,P.: On the geology and geochemistry izations. Geol. Surv. Finland, Bull. 294, 1977. 64 p. of the Vihanti zinc ore deposit, Finland. Bull. Comm, Vorma, A.: On the contact aureole of the W'iborg geol. Finlande 236, 1968. 121 p. rapakivi granite massif in southeastern Finland. Geol. Sakko, M.: Varhais-karjalaistenmetadiabaasien radiomet­ Surv. Finland, Bull. 255, 1972. 28 p. risia zirkoni-ikia, Geologi 23 (9-10),117-119.1971. - On two roof pendants in the W"iborg rapakivi Salli, I.: The structure and stratigraphy of the Yli­ southeastern Finland. Geol. Surv. Finland, vieska-Hirnanka schist area. Bull. Comm. geol. 272, 1975. 86 p. Finlande 211, 1964. 67 p. - On the petrochemistry of rapakivi granites with Sifvennoinen, A.: On the stratigraphic and structural special reference to the Laitila massif, southwestern geology of the Rukatunturi area, northeastern Finland. Finland. Geol. Surv, Finland, Bull. 285, 1976. 98 p. Geol. Surv, Finland, Bull. 257,1972. 48 p.