FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
FIFTH INTERNATIONAL DYKE CONFERENCE "Dyke swarms - time markers of crustal evolution"
Pohtimolampi Wilderness Hotel - Polar Circle - Rovaniemi - Finland 31 June – 3 August 2005
PRE-CONFERENCE FIELD TRIP A GUIDEBOOK: Southern and Central Finland
edited by Luttinen, A.V. Department of Geology P.O.Box 64, FIN-00014 University of Helsinki
Vuollo, J.I. Geological Survey of Finland P.O.Box 77 FIN-96101 Rovaniemi
ISBN 951-690-927-2 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Organising Committee Dr. Jouni Vuollo (Geological Survey of Finland) Dr. Satu Mertanen (Geological Survey of Finland) Prof. Olav Eklund (University of Turku) Prof. Eero Hanski (University of Oulu) Dr. Hannu Huhma( Geological Survey of Finland) Dr. Pentti Hölttä (Geological Survey of Finland) Dr. Arto Luttinen (University of Helsinki) Prof. Tapani Rämö (University of Helsinki) Dr. Saku Vuori (Geological Survey of Finland)
IDC5 Congress Office Ms. Marja-Leena Porsanger & Ms. Päivi Mäkikokkila (Rovaniemi-Lapland Con- gresses, University of Lapland)
Sponsors
Scientific Geological Survey of Finland Large Igneous Provinces Commission University of Oulu University of Helsinki University of Turku
Technical sponsors Geological Survey of Finland University of Oulu University of Helsinki University of Turku Tulikivi Kivikeskus
Financial sponsors Academy of Finland Geological Survey of Finland City of Rovaniemi & Municipality of Rovaniemi European Diamonds PLC Scandinavian Gold Limited Kiviteollisuusliitto
FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Table of contents
Introduction i Itinerary and stop descriptions ii
Proterozoic mafic dyke swarms of southern and central Finland 1 1. General geological background 1 2. Mafic magmatism 4 2.1. Rifting and breakup of the Archaean craton (2.5–2.0 Ga) 4 ~2.45 Ga dyke swarms 4 ~2.32 Ga dyke swarm and intrusions 5 ~2.2 Ga layered sills and dykes 5 ~2.1 Ga dyke swarms 5 ~1.98 Ga dyke swarm 6 2.2. Seafloor spreading (1.95 Ga) 6 2.3. Rapakivi-related mafic magmatism (1.7–1.4 Ga) 7 Plutons 7 Dyke swarms and basalts 7 2.4. Rifting at 1.2 Ga 8 2.5. Dyke emplacement related to Sveconorwegian orogny (~1.1 Ga) 9 2.6. Dykes related to the Caledonides (~430 Ma) 9 2.7. Palaeozoic kimberlite magmatism 10 Day 1 – STOPS 1.1. – 1.3. 11 Day 2 – STOPS 2.1. – 2.10. 15 Day 3 – STOPS 3.1. – 3.3. 23 Day 4 – STOPS 4.1. – 4.4. 26 Day 5 – STOPS 5.1. – 5.9. 37 Day 6 – STOPS 6.1. – 6.8. 53 References 59
FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Introduction
This fieldtrip was organized as a part of the 5th International Dyke Conference to be held in Rovaniemi, Finland, July 31 – August 3 (Fig. 1). The purpose of the excursion is to highlight the geology of various Proterozoic dyke systems, including diabase dyke swarms, the Åva ring com- plex, microtonalites, kimberlites, and dykes belonging to the Jormua ophiolite complex, in southern and central Finland. We depart from Helsinki downtown on July 26. On our 300 km drive to Rauma, we will first examine the ~1.65 Ga Kopparnäs dyke. In the afternoon we make two stops in order to study the so- called Postjotnian 1.2 Ga dolerite sills and dykes in the Satakunta region. We stay overnight in Rauma. On July 27, we take a ferry to the Åland archipelago. Our first target is the 500 m wide 1.6 Ga old bimodal Korsö dyke in the Åland–Åboland dyke swarm. In the afternoon we focus on the bi- modality between granite and shoshonitic magmas and related lamprophyre dykes in the post- collisional 1.8 Ga Åva ring complex. We stay overnight in Uusikaupunki. The main objective of the third day, July 28, is driving some 500 km to the east. Along our way, we make quick stops to examine the ~1.7 Ga Häme swarm; one dyke near Orivesi contains well-preserved basaltic glass. We recover from the strenuous drive in our peaceful lodging at Kivennapa. We begin our fourth excursion day, July 29, by examining ~1.85 Ga microtonalite dykes at Kivennapa and then continue with Finnnish kimberlites at Kaavi. In the afternoon, we visit the Tulikivi quarry and factory (soapstone products) in Nunnanlahti. We spend the night in Koli. On July 30, we focus on the ~2 Ga dykes in the Koli area and Varpaisjärvi. We also visit the Finnish Rock Centre at Juuka before arriving at our lodging in Kajaani. On July 31, we visit some of the key outcrops of the Early Proterozoic Jormua Ophiolite complex in the morning and then head towards Rovaniemi. A quick stop to examine the oldest rock of Europe at Siurua will make the 300 km transit more bearable. We will arrive at the conference venue, Rovaniemi, in time to participate to the IDC5 ice-breaker party at the Pohtimolampi Wilder- ness Hotel.
Arto Luttinen and Jouni Vuollo
i FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Itinerary and stops
DAY 1 - 26 JULY:
07:45 Name call at Hotel Arthur, Helsinki 08:00 Minibuses leave for Kopparnäs 11:00 Lunch at Mustio Castle 12:00 Fieldtrip in Satakunta area 18:00 Lodging and dinner at Hotel Kalatorin Majatalo, Rauma
DAY 2 - 27 JULY:
07:00 Breakfast at Kalatorin Majatalo 08:00 Minibuses leave for Åva 11:05 Ferry to Brändö, Åland 11:40 Fieldtrip and lunch on Brändö 18:30 Ferry to mainland 21:00 Lodging and dinner at Hotel Aquarius, Uusikaupunki
DAY 3 - 28 JULY:
07:00 Breakfast at Hotel Aquarius 08:00 Minibuses leave for fieldtrip 11:00 Fieldtrip and lunch in Orivesi area 18:00 Lodging, sauna, dinner, and lecture at Hotel Kivennapa
DAY 4 - 29 JULY:
07:00 Breakfast at Hotel Kivennapa 08:00 Fieldtrip starts at Kivennapa 09:00 Minibuses leave for Kaavi 09:30 Fieldtrip in the Kaavi area 14:00 Lunch and guided tour at Tulikivi, Nunnanlahti 17:30 Lodging and dinner at Hotel Koli
DAY 5 - 30 JULY: 07:00 Breakfast at Hotel Koli 08:00 Fieldtrip at Kaunisniemi 13:00 Lunch and guided tour at Finnish Rock Centre, Juuka 15:00 Fieldtrip at Varpaisjärvi 18:00 Lodging and dinner at Hotel Kajaani
DAY 6 - 31 JULY: 06:30 Breakfast at Hotel Kajaani 07:30 Minibuses leave for Jormua 08:00 Fieldtrip and lunch in Jormua region 13:00 Minibuses leave for Rovaniemi 18:00 Arrival at Conference venue 19:00 Ice-breaker party
ii FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Stop descriptions
26.7.2005 (Guide: Arto Luttinen –University of Helsinki) Stop 1.1. Kopparnäs dyke (AL) Stop 1.2. Hinnerjoki crosscutting dyke (AL) Stop 1.3. Reposaari dyke (AL)
27.7.2005 (Guide: Olav Eklund –University of Turku) Stop 2.1. Ring dykes near Björnholma (OE) Stop 2.2. Bimodal lamprophyre–granite dykes at Åva harbour (OE) Stop 2.3. Roof pendants, monzonite-granite interactions, pegmatites (OE) Stop 2.4. Thin tholiitic dykes south of Brändö (OE) Stop 2.5. Korsö dyke (OE) Stop 2.6. Leuconorite and labradorite megacrysts (OE) Stop 2.7. Vesicular granite, hybrid, and snow-flake plagioclase (OE) Stop 2.8. Gabbroic autoliths (OE Stop 2.9. Pyterlitic rapakivi granite (OE) Stop 2.10. Järppilä quartz-feldspar porphyry dyke (OE)
28.7.2005 (Guide: Arto Luttinen –University of Helsinki) Stop 3.1. Proterozoic basaltic glass in Orivesi dykes (AL) Stop 3.2. Flow-differentiated Vehkajärvi dyke (AL) Stop 3.3. Quartzite cobbles in Maijaanvuori dyke (AL)
29.7.2005 (Guides: Matti Tyni; Olli Äikäs –Geological Survey of Finland) Stop 4.1. Microtonalite dykes at Kivennapa (OÄ) Stop 4.2. Kimberlite boulders (MT) Stop 4.3. Kimberlite pipe 2 (MT) Stop 4.4. Tulikivi soapstone factory
30.7.2005 (Guides: Jouko Parviainen and Jouni Vuollo –Geological Survey of Finland) Stop 5.1. Olivine (-chromite) cumulate (JV) Stop 5.2. Layered olivine, chromite, and clinopyroxene cumulates (JV) Stop 5.3. Clinopyroxenite (JV) Stop 5.4. Magnetite clinopyroxenite and magnetite gabbro (JV) Stop 5.5. Layered and laminated magnetite gabbro (JV) Stop 5.6. Granophyre, marginal types, and basement granite (JV) Stop 5.7. The 1.97 Ga dyke (JV) Stop 5.8. The Finnish Stone Centre, Juuka Stop 5.9. Varpaisjärvi dyke (JP)
31.7.2005 (Guides: Hannu Huhma, Asko Kontinen and Katja Lalli –Geological Survey of Finland) Stop 6.1. Clinopyroxenite and hornblendite-garnetite dykes (AK) Stop 6.2. “OIB” metalamprophyre and “E-MORB” dykes (AK) Stop 6.3. Sheeted dykes (AK) Stop 6.4. Harzburgite and metarodingitic gabbro dyke (AK) Stop 6.5. Gabbros, ferrogabros and plagiogranites (AK) Stop 6.6. Podiform chromitite (AK) Stop 6.7. Pillow lavas (AK) Stop 6.8. The oldest rock of Europe at Siurua (HH, KL)
iii FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Figure 1. General geology of southern and central Finland showing the areas to be visited. Red star depicts the conference venue at Hotel Pohtimolampi.
iv FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Proterozoic mafic dyke swarms of southern and central Finland
1. General geological background
The Finnish bedrock makes up roughly one third of the Fennoscandian Shield. The bedrock is divided into an Archaean complex (3.50–2.50 Ga) in the north and east and an Early Proterozoic (1.92–1.77 Ga) Svecofennian domain which dominates in the central and southern Finland (Fig. 1). The Archaean basement is mainly composed of ~2.7 Ga granodiorites and tonalities and interfingered ~2.8 Ga greenstone belts. The 2.58 Ga Siilinjärvi carbonatite complex near Kuopio is among the oldest carbonatites world-wide. A sequence of continental sediments and lavas, designated as the Karelian Supergroup, was deposited on the Archaean basement during successive periods of rifting within the Archaean craton. At the base there are felsic lavas (Sumi), followed by immature conglomerates, sandstones and intermediate volcanites (Sariola). Subsequently, a thick aluminium-rich weathered crust was developed, which indicates low-latitude palaeoposition of the Archaean continent. The hiatus is overlain by recycled weathered material and quartz-rich sediments (Kainuu) which were deposited in fluvial systems in small grabens and on the flanks of Archaean crustal blocks. Advanced rifting led to sedimentation of up to 3–4 km thick quartz-rich sandstones in fluvial and shallow marine environments (Jatuli) and, finally, marine clay-rich sandstones, clays, iron formations, and metalligenous black schists (Kaleva). The Archaean basement and the sedimentary cover were repeatedly intruded by mafic plutons and dykes during the Early Proterozoic time (2.50–1.98 Ga). Based on current geochronological data, the intrusions may represent six, or possibly seven, distinctive magmatic events and indicate periodic intracontinental rifting with intervals of ~50–100 Ma between subsequent rifting events. These rifting events culminated in the formation of a passive continental margin as the Archaean craton broke up at ~2 Ga. The now-separated “western” cratonic block has not been identified, although Archaean blocks in northern Sweden may represent reassembled parts of it. Obducted remnants of oceanic crust (eg. 1.95 Ga Jormua ophiolite) manifest a transition from continental rift-related magmatism to seafloor spreading after the break up. Isotope geological studies have established a suture between the Archaean and the Svecofennian domains. The origin and evolution of the 1.9–1.8 Ga Svecofennian orogenic crust has remained somewhat controversial. The Svecofennian domain of central and southern Finland has been traditionally divided into three tectonic units based on lithological criteria and U-Pb zircon age data. These units form 1) the primitive arc complex of central Finland adjacent to the Archaean craton, 2) the accretionary arc complex of central and western Finland, and 3) the accretionary arc complex of southern Finland (e.g. Korsman et al., 1997; Nironen, 1997) (Fig. 2). Much of the accretionary arc complex of central and western Finland is occupied by the 40 000 km2 Central Finland Granitoid Complex (Elliott et al., 1998; Nironen et al., 2000). The generally accepted concept of a semi-continuous Svecokarelian/Svecofennian orogeny has been recently rejected by Lahtinen et al. (2003) who have put forward a more complex orogenic model for the Early Proterozoic evolution of the Svecofennian domain. The model involves >2.0 Ga microcontinents, pre-1.92 Ga island arcs, and 1.90–1.82 arcs that accreted to the Archaean craton to form the Fennoscandian Shield. The following pre-1.92 Ga components have been proposed: Karelian, Kola and Norrbotten Archaean cratons; Keitele, Bergslagen and Bothnia >2.0 Ga microcontinents; Kittilä ~2.0 Ga island arc; and Savo, Knaften, Inari and Tersk ~1.95 Ga island arcs. The assembly of the cratons and microcontinents at 1.92–1.88 Ga to form a larger Fennoscandian continental plate was followed by a continental extensional stage along the Archaean-Proterozoic boundary and in the central part of the continent at 1.88–1.85 Ga. Collisions of Fennoscandia with Sarmatia and Amazonia have been envisaged at 1.85–1.79 Ga. The orogenic cycle was terminated by an orogenic collapse and stabilization stage (1.79–1.77 Ga).
1 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 2. Schematic map of the Finnish bedrock (Korsman et al., 1999)
The Middle Proterozoic (1.65–1.54 Ga) rapakivi granites of southern Finland represent the predominant post-orogenic rock type. Little is known about the ~100 Ma interlude between the Early Proterozoic orogenic stage and the emplacement of the bimodal rapakivi associations. The Svecofennian mountain chain had already been eroded down to its roots when processes initiated in the Earth’s mantle within Fennoscandia led to the generation and ascent of voluminous mafic magmas and anatectic rapakivi magmas into the crust and to the surface. The emplacement of rapakivi associations was periodic and took place between 1.65 and 1.53 Ga; some of the diabase dykes may be marginally older at 1.67 Ga. The principal magma source of the rapakivi plutons has been ascribed to lower Svecofennian crust (Rämö, 1991). Mixing and mingling of mafic and felsic magmas has been limited on large scale apart from the Jaala-Iitti complex (Salonsaari, 1995).
2 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 3. General geology of southern and central Finland showing the distribution of ~2.5 Ga to 0.4 Ga intrusions. The pre- and post-symposium excursion routes are indicated by blue and red lines, respectively. Red stars depict excursion stops.
3 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
2. Mafic magmatism
The mafic dyke swarms in Finland have traditionally been divided into three main groups (e.g., Aro & Laitakari, 1987): (1) the Jatulian diabase dykes and sills (>1.9 Ga), (2) the Subjotnian diabase dykes (~1.6 Ga), and (3) the Postjotnian diabase dykes (~1.3 Ga) (Fig. 3). The Jatulian magmatism was associated with multiple rifting episodes which culminated in the break up of the Archaean craton. The onset of rifting was manifested by the emplacement of mafic dykes and voluminous layered intrusions at 2.50 Ga and 2.45 Ga in eastern and northern Finland and in the Kola Peninsula. Dykes and plutons which have been dated at 2.32 Ga, 2.2 Ga, 2.1 Ga, 2.05 Ga, and 1.98 Ga have been associated with a sequence of intrusive events which eventually led to the generation of a passive continental margin. Obducted remnants of ~1.95 Ga oceanic lithosphere in Jormua provide an insight into a period of seafloor spreading which was terminated by the accretion of juvenile volcanic arcs to the rifted margin of the Archaean craton at ~1.9 Ga. During the Svecofennian orogen (1.9–1.8 Ga), several lithospheric terranes were amalgamated and provided the essential building blocks of the Fennoscandian Shield. After some ~100 Ma of erosion, a distinctive period of rifting and anorogenic bimodal magmatism commenced at 1.7 Ga and lasted until 1.4 Ga. The last major period of crustal growth took place at ~1.3 Ga when the Jotnian sedimentary basins were intruded by voluminous diabase sills and dykes. This event and the subsequent intrusion of ~1 Ga diabase dykes in the northern Finland (e.g. Salla dyke; site 7.3. post-symposium excursion) may have originated in response to the Sveconorwegian orogeny.
2.1. Rifting and breakup of the Archaean craton (2.5–2.0 Ga)
Several episodes of dyke emplacement during the period 2.5–1.98 Ga have been indicated by U-Pb zircon geochronology (cf. Vuollo, 1994). The following groups have been identified based on age data, geochemical composition, and mode of occurrence: 2.5, 2.45, 2.32, 2.2, 2.1, and 1.98 Ga. The oldest group is only found in the Kola Peninsula, whereas the others are found across the Karelian province. The ~2.45 Ga dyke swarms can be divided into five subgroups based on their field relationships and geochemical and isotopic characteristics: (1) NE-trending boninite-noritic dykes, (2) NW-trending gabbronorite dykes, (3) NW-trending tholeiitic dykes, (4) NW- and E- trending Fe-tholeiitic dykes, and (5) E-trending orthopyroxene-plagioclase phyric dykes. The younger dyke swarms (2.32–1.98 Ga) form a homogeneous group in terms of geochemical composition and, in general, resemble continental tholeiitic basalts.
~2.45 Ga dyke swarms
The most conspicuous products of the ~2.45 Ga magmatism are undoubtedly the mafic layered intrusions, which are known for their Cr and PGE ores and include the Tornio- Näränkävaara belt and the Koitelainen and Akanvaara intrusions in Finland and also the Oulanka complex (Lukkulaisvaara, Tsipringa, and Kivakka intrusions) near the Finnish border in Russia, and the Burakovsky intrusion east of Lake Onega (Alapieti et al., 1990). Mafic dykes of the same age are also found throughout the Archaean areas. One distinctive feature of the 2.45 dyke swarms compared with the younger dyke swarms is their geochemical variability. The boninite-norite dykes, the gabbronorite dykes, and the orthopyroxene-plagioclase phyric dykes have been grouped as the BN dykes based on their calc-alkaline affinities and a general compositional similarity. The BN dykes have much higher SiO2 (50–57 %) and generally higher Mg-number than the tholeiitic dykes. They are characterised by variably high LREE/HREE ratios, with (La/Yb)N of 3–11. Results of Sm-Nd isotopic studies show that the BN dykes (no data for the orthopyroxene-plagioclase
4 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
phyric dykes are available) have negative initial ,Nd values (– 2.4 to –1.8), which correspond to values previously obtained for the layered mafic intrusions (Huhma et al., 1990; Turchenko et al., 1991; Amelin et al., 1996; Saini-Eidukat et al., 1997; Hanski et al., 2001). The tholeiitic dykes are usually slightly LREE-enriched, but may also show flat or LREE-depleted patterns. Isotopic data for the tholeiites show slightly positive initial ,Nd values (+0.3 to +1.7). These results suggest that the ~2.45 Ga dykes may be consanguineous with the 2.45 Ga intrusions.
~2.32 Ga dyke swarm and intrusions
Until recently, there have been few indications of ~2.3 Ga magmatic events in the eastern part of the Fennoscandian Shield (Paavola, 1988). However, Huhma et al. (1990) obtained a Sm- Nd isochron age of 2330±180 Ma for the Runkaus metalavas in the Peräpohja area and new unpublished age determinations (Vuollo et al., 2000; Juha Nykänen, pers. comm., 2003; Jorma Paavola, pers. comm., 2003) indicate that the ~2.32 Ga magmatism represents a significant magmatic event in the eastern part of the shield. The areal distribution of the 2.32 Ga dykes and intrusions is difficult to estimate because of few available ages and the fact that these igneous rocks are geochemically inconspicuous.
~2.2 Ga layered sills and dykes
The ~2.2 Ga “karjalitic” layered sills and intrusions form a conspicuous mafic-ultramafic magmatic suite that is wide-spread in the eastern and northern parts of Finland, including all the Karelian schist belts. These differentiated sills and intrusions have been called by a variety of names: karjalite (Väyrynen, 1938; Vuollo & Piirainen, 1992), hypabyssal spilite (Piirainen 1969), and the gabbro-wehrlite association (Hanski, 1986). Various alternatives have also been proposed for the parental magma, including olivine basalt (Meriläinen, 1961), tholeiite (Piirainen, 1969), Fe- picrite (Hanski, 1986), and low-Al tholeiite (Vuollo & Piirainen, 1992). Age determinations for karjalitic sills from Peräpohja, central Lapland, Kuusamo, Kuhmo, and North Karelia allow us to conclude that the emplacement of the karjalitic sills and dykes took place at ~2220 Ma. Stratigraphically, the 2.2 Ga sills and intrusions are restricted to the vicinity of the unconformity between the Archaean basement and Early Proterozoic metasediments, and they are found within both of them. Most often they lie parallel to the overlying Karelian metasediment series (Karelian belt) and are thus referred to as sills. Most typically, they are found as sills varying from a few kilometres up to 150 km in length and are quite thin (200–400 m) relative to their length. Later tectonic movements have fragmented the originally continuous intrusions. As exemplified by the Koli layered sill, the karjalitic intrusions contain only one magmatic cycle and were crystallized from a single magma pulse that crystallized into a highly differentiated structure with a simple internal stratigraphy. Geochemically, the karjalitic sills form a peculiar magmatic series. The composition of the cumulates, granophyre, and the chilled margin is consistent with an overall LREE-enriched character of the parental magma. This was a low-Al tholeiite (~10 % Al2O3) rich in iron (~13 % FeOtot) and LREE [(La/Yb)N = 5.8]. A low Al2O3/TiO2 ratio of 5 to 6 is also a characteristic feature of the 2.2 Ga layered sills.
~2.1 Ga dyke swarms
Emplacement of the ~2.1 Ga Fe-tholeiitic dykes was a wide-spread magmatic event that affected all parts of central and northern Finland. It gave rise to a dense, predominantly NW- and E-trending dyke network that now intersects the Karelian formations and the Archaean basement (Fig. 3). New ages from the Kuhmo block and central Lapland show that there were many stages of
5 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
dyke intrusion between 2.1 and 1.98 Ga. Regional differences can be observed; e.g., the nortwesterly orientation is dominant in the North Karelia and Kainuu schist belts, whereas in the Archaean basement of the Kuhmo block, the main orientation is to the east (Anttila et al., 1991; Kilpelä, 1991). The scarcity of outcrops mean that the distribution of the dykes in Lapland must be estimated based on their stratigraphic position and geochemistry. Available material suggests that the Fe-tholeiitic dykes are preponderant also in Lapland. In the Archaean basement (Kuhmo block), the Fe-tholeiitic dykes form a highly regular swarm in which individual dykes vary from a few centimeters up to 200 m in width. They can be followed for a few hundred meters to several tens of kilometers along the strike. On a large scale, the dyke swarms have an ‘en echelon’ structure. The dykes are homogeneous in their modal composition, contain principally hornblende and plagioclase, and have been referred to as metadiabases (e.g., Piirainen, 1969; Pekkarinen, 1979). In the northern part of the Kuhmo block (on the eastern side of the Kuhmo greenstone belt), these ~2.1 Ga Fe-tholeiitic dykes have preserved their primary mineral composition. Geochemically, the Fe-tholeiitic dykes form a relatively homogeneous group from North Karelia through Kainuu to Lapland (Vuollo et al., 1992; Vuollo, 1994). They are quartz-normative, sub-alkaline tholeiitic basalts and form a set of continental dyke swarms of the type frequently found in shield areas. Several U-Pb ages from North Karelia, Varpaisjärvi, and Kuusamo point to an overall age of ~2.1 Ga for the emplacementof the Fe-tholeiitic dykes. New U-Pb ages from central Lapland (Rastas et al., 2001; Räsänen & Huhma, 2001) show that a significant magmatic event occurred also at ~2.05 Ga. This event is also registered by the Keivitsa and Otanmäki intrusions.
~1.98 Ga dyke swarm
Previous studies (Vuollo, 1994) show that the ~1.98 Ga dyke swarm is not as voluminous as the ~2.1 Ga dyke swarm. However, recent geochronological (Vuollo & Huhma, 2004) and field studies, combined with aeromagnetic data, show that the ~1.98 Ga dykes are found throughout the Archean basement and the Karelian formations (Fig. 3). The swarm consists of up to 70 m wide, coarse- to medium-grained, ophitic dykes that form prominent NW-trending, linear features >120 km in length (Kuhmo block). The 1.98 Ga dyke swarm is the youngest and least deformed Early Proterozoic mafic dyke swarm observed within the Fennoscandian Archean craton and is closely connected to the 1.95 Ga ophiolites (Kontinen, 1987; Vuollo et al., 1992; Hanski, 1997). A U-Pb age data are somewhat scattered (1972±5 Ma to 1995±9 Ma; Vuollo et al., 1992 and Rastas et. al., 2001, respectively) but show that the ~1.98 Ga swarm is present throughout the eastern part of the Fennoscandian Shield. According to Vuollo et al. (1992), the 1.98 Ga dykes are Fe-tholeiitic to tholeiitic in composition, have continental affinities, and are geochemically indistinguishable from the older tholeiitic dyke swarms. The 1.98–1.95Ga mafic magmatic events were extremely significant for the ore-forming processes of the Fennoscandian Shield, as indicated by the ore- bearing ophiolite formations (a breakup event at ~2.0 Ga) in Finland.
2.2. Seafloor spreading (1.95 Ga)
Opinions differ with regard to the exact timing of the final break up of the Archaean Craton. At 1.95 Ga, the continental rifting and associated magmatism had given way to seafloor spreading. The Jormua ophiolite complex (Kontinen, 1987; Peltonen et al., 1996) is a rare example of Early Proterozoic oceanic lithosphere. It consists of two 1–4 km wide and >25 km long tectonic blocks that interfinger with the Archaean basement and its Early Proterozoic dedimentary cover. The ~1000 m thick sequence includes most of the stratigraphic parts of oceanic lithosphere including: 1) serpentinite, 2) gabbroic intrusions, 3) sheeted dykes, 4) pillow lavas, and 5) pelagic sediments. Metatronhjemites akin to modern oceanic granitoids have also been found at Jormua. The
6 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
serpentinites host various mafic dykes, some of which exhibit affinities to ocean island basalts (OIB).
2.3. Rapakivi-related mafic magmatism (1.7–1.4 Ga)
Large volumes of mafic magma intruded the Svecofennian crust during the rapakivi magmatic period at 1.7–1.4 Ga. Only a few pluton-size intrusions are visible at the current erosion level, whereas rapakivi-associated dykes have been found across southern Finland. They are most conspicious to the northwest of the Wiborg rapakivi massif and in the Åland archipelago where they comprise distinctive swarms.
Plutons
The majority of the mafic rapakivi-related plutonic rocks are leucocratic gabbros. Troctolitic and anorthositic varieties have also been discovered. The mafic plutonic rocks are found either as inclusions within the rapakivi plutons or as small intrusions on their margins. Geochronological studies indicate indistinguishable intrusion ages for the felsic and mafic plutonic rocks of the rapakivi areas (Suominen, 1991; Vaasjoki et al., 1991; Alviola et al., 1999). The Ahvenisto gabbro–anorthosite complex is the largest of the exposed plutons and surrounds the Ahvenisto rapakivi pluton at Mäntyharju northwest of the Wiborg rapakivi area. The complex is typified by coarse-grained leucogabbronorite, which contains scattered large (up to 10 cm) plagioclase crystals. Olivine-bearing mafic plutonic rocks (olivine leucogabbronorite and troctolite) are found in the northwestern part of the complex. Anorthosite is found as lens-shaped, well-defined (sharp contacts) up to 200 m long bodies in the more mafic rock types of the complex. Recent seismic studies indicate a voluminous gabbro–anorthosite pluton beneath the Wiborg batholith (Korja, 1995). Based on magnetic and gravimetric surveys, Elo et al. (1996) have suggested that the Ahvenisto gabbro–anorthosite complex is a small outcropping part of this pluton. Relatively small rapakivi-associated mafic plutonic rocks are also found (~0.2 – ~2 km) within the central part of the Wiborg batholith and are associated with the Suomenniemi pluton, the Laitila batholith, and the Åland batholith. Monzodiorites, quartz monzodiorites, and dykes from the Ahvenisto gabbro–anorthosite complex (Johanson, 1984; Alviola et al., 1999) are rare examples of rapakivi-associated intermediate rock types. Based on cross-cutting relationships, the intermediate dykes are younger than the gabbros and anorthosites, but older than the rapakivi granites. Intermediate rock types have been also described from the Åland batholith (Eklund et al., 1994).
Dyke swarms and basalts
Evidence for rapakivi-related volcanic activity is found on the Island of Suursaari (Hogland), near the southern margin of the Wiborg massif. The Early Proterozoic Svecofennian bedrock is discordantly overlain by an undeformed conglomerate, a thin layer of pyroclastic material, basaltic and andesitic lavas, and a second layer of pyroclastic material underneath a capping, over 100 m thick unit of quartz-feldspar porphyries. Rapakivi-related dyke rocks are wide-spread in southern Finland (Fig. 4). Northwest of the Suomenniemi rapakivi pluton is a NW-trending, over 80 km long swarm of tholeiitic diabase dykes (Rämö, 1991). Some of the intrusions belonging to the Suomenniemi dyke swarm contain alkali feldspar ovoids, which suggests that the magmas have passed through a silicic magma chamber. Locally, mingling structures indicate passage of mafic and felsic magmas through the same fissures. The 1.65 Ga Lovasjärvi mafic intrusion is a ~5 km long and 800–1500 m wide vertical sheet-like body that is composed of diabase, olivine diabase, and melatroctolite (Alviola, 1981;
7 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Siivola, 1987). The Häme dyke swarm extends ~150 km NW of the Wiborg rapakivi batholith. Two sets of dykes have been identified based on different strikes and compositions, and geochronological data. The older 1665 Ma old (Vaasjoki & Sakko, 1989) set is ~100 km long, strikes west-northwest, and is typified by abundant olivine (Laitakari, 1987). The younger ~1645 Ma old set is ~150 km long, strikes northwest, and is typified by phenocrysts, megacrysts (up to <20 cm), and fragments of plagioclase (Laitakari, 1969). The younger dykes are mainly olivine tholeiites, but they are more evolved than the older dykes and include also quartz tholeiites. Noncrystalline glass has been discovered in the tails of some dykes (Lindqvist & Laitakari, 1980). Mafic dyke rocks have been found near the small rapakivi plutons around Helsinki (Onas, Bodom, Obbnäs) (e.g. Törnroos, 1984; Kosunen, 1999). Correlation of these undated dykes with the rapakivi granites and the Häme and Suoenniemi swarms is not straightforward, but geochemical and palaeomagnetic data (Satu Mertanen, personal communication, 2005) lend support to the interpretation that they were emplaced coevally with the rapakivi granite magmas. Numerous mafic dyke rocks between Åland and Turku (Ehlers & Ehlers, 1977; Suominen, 1987, 1991), in the southwestern part of the Åland rapakivi batholith (Eklund, 1993), in the Laitila batholith (Haapala, 1977), and near the town of Pori north of Laitila (Pihlaja, 1987) are believed to be coeval with rapakivi granite plutons. One of the best known examples is the 35 km long Föglö swarm between the Åland and Kökarsfjärden rapakivi plutons. The en echelon dykes have been dated at ~1570–1540 Ma. Similar to the younger set of diabase dykes in the Häme swarm, many of the Föglö dykes contain large plagioclase fragments, even several tens of centimeters in diameter.
Figure 4. Distribution of rapakivi granites and Subjotnian diabase dyke swarms in southern Finland.
2.4. Rifting at 1.2 Ga
The emplacement of bimodal rapakivi associations was the latest significant period of crustal growth in Finland and the Fennoscandian Shield in general. Evidece for younger igneous activity is sparse. The late evolution, which has been influenced by the Sveconorwegian and Caledonian orogenies, is divided into distinctive phases on the basis of characteristic rock record and tectonic significance. After the rapakivi magmatic cycle, thick fluvial deposits began to fill the developing intracratonic rift basins. Remnants of these so-called Jotnian sequences are preserved in the tectonic depressions and graben structures at Satakunta and Muhos. The intrusion of diabase sills and dykes into the Jotnian sedimentary rocks at 1265 Ma is the most important post-rapakivi
8 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
magmatic event. These “Postjotnian” intrusions represent the eastern part of a wider ~1300 Ma mafic igneous province that extends to central Sweden. The origin of this province has been associated with initial crustal extension of the Sveconorwegian orogeny (~1300–900 Ma). In the Satakunta region, extensive, vertical to subhorizontal ~1265 Ma “Postjotnian” dykes cut the Svecofennian granitoids and metamorphic rocks, the rapakivi granites, and the Satakunta sandstone. Mafic hypabyssal rocks of this age group (Suominen, 1991; Vaasjoki, 1996) are also exposed in the Åland region (Bergman, 1979) and in the Vaasa region (Aro, 1987). The sub- horizontal dykes are usually several tens of meters thick and some contain megaophitic parts. Geochemically, they are fairly evolved transitional between olivine tholeiites and alkali basalts. Isotopic analyses indicate radiogenic Nd compositions with initial ,Nd (1265 Ma) values from +1.3 to +3.5 and depleted mantle model ages (DePaolo, 1981) of 1.71–1.44 Ga. It is not clear whether the lower ,Nd values of the dykes reflect crustal contamination associated with low-pressure fractional crystallization (cf. Rämö, 1990) or indicate mantle source compositions (cf. Patchett et al., 1994).
2.5. Dyke emplacement related to Sveconorwegian orogny (~1.1 Ga)
The ~1.1–1.0 Ga diabase dykes of Lapland manifest igneous activity that followed the cratonisation of the Fennoscandian Shield. Their origin is temporally, and possibly also petrogenetically, related to the closing stages of the Sveconorwegian orogey (Jarmo Kohonen, personal communication, 2005). The diabases that crosscut the Salla greenstone belt area are among the youngest igneous rocks in Finland. They are cutting all the volcanites, metasediments and granitoids. The Salla dyke swarm can be followed laterally ~150 km across the boarder from Russia (Tuutijärvi) to Salla and at least to village of Sodankylä. The en echelon Salla dyke is nearly vertical and highly differentiated. The width varies from 60 to 100 m. The age of the Salla dyke is 1122±5 Ma (Lauerma 1995). Other, presumably slightly younger swarms of basaltic dykes are found in the far northern part of the country and in adjacent Norway (Fig. 5). Mertanen et al. (1996) reported Sm-Nd whole- rock mineral ages of ~1050 Ma for samples belonging to the Laanila and Kautokeino diabases. The en echelon Laanila dyke strikes NE and extends for >100 km from Laanila to Ristijärvi in Finland and into northeastern Norway (Fig. 5) In the southwest, the diabases cut the Early Proterozoic Lapland granulite belt and in the northeast, in Ristijärvi, they cut the Archaean basement. The granulite belt is a high-grade metamorphic belt overthrust onto the Karelian basement in the southwest. The Kautokeino dykes can be followed for ~50 km on low altitude aeromagnetic maps. Individual dykes are 20–50 m wide and their length ranges from hundreds of metres to tens of kilometres. Geochemical data for the 1100–1000 Ma mafic dykes are relatively few. According to Pihlaja (1987), the Laanila dyke is an olivine tholeiite with 46 to 49.% SiO2 and 5.4–6.8% MgO. The Nd isotopic data of Mertanen et al. (1996) indicate notably positive initial ,Nd values (+4.8 to +5.8) for the Laanila and Kautokeino dykes. Given that the diabase magmas transected an Early Proterozoic–Archaean crustal domain, the effect of crustal contamination on the Nd isotopic compositions has apparently been minimal.
2.7. Dykes related to the Caledonides (~430 Ma)
In the northwesternmost reaches of Finland, dykes belonging to the Caledonides have been found. One baddeleyite age determination (Sipilä, 1992) yielded an age of 434 Ma. Dykes of this age group are mainly exposed outside Finland.
9 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
2.8. Palaeozoic kimberlite magmatism
More than twenty-four intrusive bodies, mostly kimberlitic diatremes, but also including hypabyssal kimberlites, olivine lamproites and ultramafic lamprophyres have been discovered within the Archaean Karelian Craton in Finland. These Paleozoic intrusions, two pipes have K-Ar ages of 590 and 430 Ma (Tyni, 1997), form the Eastern Finland Kimberlite Province which includes the Kaavi and Kuopio clusters. The pipes and dikes intruded into 3.1–2.6 Ga gneiss complexes of the Archaean Karelian craton and allochthonous 1.9–1.8 Ga metasedimentary cover rocks thrust onto the craton during the Svecofennian orogeny (Kontinen et al., 1992). The intrusions are dominated by textural and mineralogical variants of archetypal kimberlites and range from purely hypabyssal kimberlite dikes to multiphase pipes of diatreme facies rocks. The Finnish kimberlites generally contain abundant diamond indicator minerals and almost all contain at least trace amounts of microdiamonds. In addition to the kimberlites, a spatially distinct and presumably older group of dikes located elsewhere in the Karelian Craton comprise ultramafic lamprophyre and rocks that have both olivine lamproite and Group II kimberlite affinities.
Figure 5. Generalised geological map of the northern Fennoscandian Shield showing the distribution of ~1 Ga Laanila and Kautokeino diabases (Mertanen et al., 1996).
10 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Day 1: Subjotnian Kopparnäs dyke and Postjotnian diabases
Subjotnian diabase dykes
Arto Luttinen Department of Geology, P.O.Box 64, FI-00014 University of Helsinki
A fairly continuous swarm of diabase dykes stretches ~300 km from Lappeenranta, southeast Finland, to the north of Tampere, central southern Finland (Fig. 4). The diabases are undeformed, crosscut the Svecofennian gneisses, and are collectively designated as Subjotnian diabase dykes. Although the dykes clearly cluster in two subsets, known as the Suomenniemi swarm and the Häme swarm, which extend ÑW of the Wiborg rapakivi massif in a radial fashion, the actual boundaries of the Subjotnian dyke system are somewhat ambiguous; diabase dykes of the same type are found sporadically in nearly all directions around it. Some of the ~2 cm up to ~1 km wide individual dykes can be followed laterally up to >10 km. Two dykes, representing NNW and NW subsets of the Häme swarm, have been dated at 1667±9 Ma (Vaasjoki & Sakko, 1989) and 1646±6 Ma (Laitakari, 1987), respectively. The Häme swarm has been petrografically studied in great detail by Laitakari (1969), but geochemical data (trace elements and isotopes in particular) are sparse, apart from the Vehkajärvi dyke (see STOP 3.2.). The existing data indicate subalkaline compositions with quartz and olivine tholeiitic affinities with TiO2 ranging from 1.0 to 3.3 %. Most of the analysed dykes are fairly evolved with MgO of 8.3–3.9. The so-called spotted olivine diabase has unusually high MgO of 12.4 %, but this probably reflects accumulation of olivine which is ~Fo75, based on 2V- measurements. Data on Nd isotopic ratios for two dykes indicate slightly radiogenic to marginally non-radiogenic initial compositions (,Nd +0.6 to –0.2) (Rämö, 1990). The Suomenniemi swarm has been studied in some detail by Rämö (1991). They are in many respects similar to the Häme dykes; i.e. evolved tholeiites with TiO2 ranging from 1.8 to 3.2 % and MgO from 5.6 to 2.3 %. They are enriched in LREE (La/Yb)N ~6) and other highly incompatible trace elements, but, as in many continental tholeiites, the Nb/La ratios are fractionated (~0.3–0.6) giving the dykes a “lithospheric affinity”. Most of the dykes have marginally non-radiogenic Nd ratios (–0.2 to –1.2), but a few of them have radiogenic Nd and initial ,Nd up to +1.6.
STOP 1.1. The Kopparnäs dyke
The Kopparnäs basalt dyke is exposed over a distance of 2 km in the coast of the Gulf of Finland ~5 km west of the Obbnäs rapakivi granite intrusion (Fig. 4). The dyke is composed of numerous en echelon segments; the overall strike of the near vertical dyke is E-W. The maximum width of the generally lens-shaped segments is ca. 1.5 m. The Kopparnäs dyke cross-cuts various granitoids, gneisses, and gabbroids. Inclusions of wall-rock are fairly abundant and probably mainly represent dislocated bridges. Recent palaeomagnetic measurements (Satu Mertanen, personal communication, 2005) imply that the undated dyke is one of the wide-spread ~1.64 Ga mafic dykes (Häme swarm) which are associated with rapakivi magmatism in southern Finland. The Kopparnäs dyke is in mainly aphanitic and aphyric. Some wide segments have a porphyritic appearance due to weakly oriented plagioclase laths. Geochemically, the dyke is notably uniform and broadly similar to the dykes which belong to the Häme swarm, but it can be distinguished from them on the basis of unusually low SiO2 (~45.6 %) and high FeOtot (~17 %) at MgO of ~4.8 %. Although the dykes plot within the alkaline field in TAS diagram, normative hypersthene and olivine indicate an olivine tholeiitic affinity. Concentrations of incompatible elements, such as Nb (42 ppm) and TiO2 (4.8 %) are the highest so far reported for rapakivi-related magmas. High (La/Sm)N (2.4), (Sm/Lu)N (3.7), and (Nb/La)N values (0.7) produce a fairly smooth
11 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
mantle-normalised incompatible element diagram that is not typical of the Häme swarm and resembles that of modern OIB. Preliminary Nd isotopic data show marginally negative initial ,Nd (1.64 Ga) values (~ –1) which are consistent with a hotspot source, but do not preclude crustal contamination. The geochemical OIB-affinities of the Kopparnäs dyke concur with models that associate the bimodal rapakivi suite of southern Finland with hotspot activity beneath the Mesoproterozoic continent.
Sampling is only allowed with permission from the excursion leader! The wet rock face can be extremely slippery –watch out!
Postjotnian diabase dykes
Arto Luttinen Department of Geology, P.O.Box 64, FI-00014 University of Helsinki
The Postjotnian diabases were intruded into a sandstone-filled graben in southwestern Finland during a relatively short period of magmatism between 1.25 and 1.27 Ga (Patchett et al., 1981; Suominen, 1987). They are found as extensive, partly horizontal dykes cutting yje Laitila rapakivi granite and the Jotnian sandstone in the graben (Hämäläinen, 1987) (Fig. 4). Adjacent to intrusive contacts, rheomorphic dykes and partially melted xenoliths manifest interaction and contamination of basaltic magma with wall-rock material. The geochemically rather uniform Postjotnian diabases are transitional between alkali basalts and olivine tholeiites; the amount of normative hy is typically <10 %. SiO2 ranges from 46 to 48 % and the diabases plot almost invariably within the alkaline field in TAS-diagram. Similar to the older Subjotnian diabases, most of the Postjotnian intrusions are fairly evolved with MgO of 7.7–4.6 % and TiO2 of 1.4–3.7 %, apart from coarse- grained patches in which SiO2 is up to 59 % and TiO2 up to 5 %. The dykes show variable (La/Sm)N (0.9–2.6). The La concentration level is low (typically 7–30 ppm) in comparison with that of Subjotnian diabases (typically 30–50 ppm). Published and unpublished Nd isotopic data indicate radiogenic initial compositions with ,Nd from +1.5 to +3.6 (Rämö, 1990; Rämö, personal communication, 2005).
STOP 1.2. The Hinnerjoki crosscutting dyke
Geological mapping and aeromagnetic images indicate fairly abundant, roughly N-S trending dykes that crosscut the Laitila rapakivi batholith and the Postjotnian diabase dykes and sills in the Satakunta region. Several of such crosscutting dykes are exposed in a road cut some 4 km east of Hinnerjoki village (Fig. 6). Age data for these dykes are lacking, but unpublished elemental geochemical and isotope geochemical analyses (Tapani Rämö, personal communication, 2005) show similarities to the 1.25 Ga dykes and sills. In the road cut, a half a dozen up to ~30 cm wide dykes crosscut a medium-grained Postjotnian sill. One of such dykes have been analysed for major and trace elements and Nd isotopic ratios. The dyke is highly evolved, marginally ne-normative alkali basalt, with MgO of 3.3 %, TiO2 of 3.4 %, and total alkalis of 5.9 %. Concentrations of incompatible elements, such as P2O5 (0.93 %), Nb (20 ppm), Zr (338 ppm), and La (36 ppm) are high and those of Ni and Cr low (7 and 5 ppm, respectively). Low Ti/Zr (60) suggests fractionation of Fe-Ti oxides. The radiogenic initial Nd isotopic composition (,Nd at 1265 Ma= +1.8) is indistinguishable from those of the Satakunta diabases (+1.4 to +3.6) and indicates a depleted source component. Low Nb/La (0.6) is similar to those of Satakunta diabase dykes and infers a lithospheric component, however.
12 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 6. Distribution of Postjotnian diabases, Jotnian sandstone, and rapakivi granite in Satakunta area (Hämäläinen, 1987). Excursion sites are indicated.
STOP 1.3. The Reposaari dyke
Postjotnian diabase crosscuts and overlies the rapakivi granites of the Eurajoki stock and Laitila batholith and the Jotnian sandstone on Reposaari, Satakunta (Fig. 6). The diabases are mainly coarse-grained, dark grayish of brownish black dykes and sills. Megaophitic varieties and diabase pegmatoids occur. Heating by diabase magma melted parts of the host rock at the contacts. In some places, the melt formed rheomorphic (palincenic) dykes in joints of already solidified diabase. In other places, the rheomorphic melt has mixed with the diabase melt, forming hybrid rocks of different types.
13 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
The Siikaranta outcrop on Reposaari is composed of rapakivi granite and diabase (Fig. 7). The intrusive contact between the diabase and the granite is exposed near the shoreline. Close to the contact, there are rheomorphic dykes and partially melted granite xenoliths. In the southwestern part of the outcrop (A in Fig. 7), the diabase is homogeneous and medium-grained. Patches of coarse-grained megaophitic portions occur here and there irregularly. A fine-grained aphanitic diabase with small granite xenoliths suggests proximity to a contact with granite (B). In the central part of the outcrop, small hybridized granite xenoliths (C) can be seen, unless the sea level is high. The rapakivi granite (D & E) contains subspherical potassium feldspar phenocrysts (2–3 cm). Rheomorphic dykes, from 1 to 10 cm wide, are found near the contact (F, G, H) in the central and northwestern parts of the outcrop. Geochemically, the Reposaari dyke is a typical Postjotnian diabase. It is marginally alkaline (ne-normative) basalt with MgO of 6.2 %, FeO2O3tot of 15.1 %, and Al2O3 of 16.7 %. The initial ,Nd value is +1.7 and well within the range of values for other Satakunta diabases.
Figure 7. Scematic map of the contact between Postjotnian diabase and rapakivi granite on Reposaari. See text for explanation for A through H.
14 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Day 2: Åva ring complex and Korsö dyke on Brändö, Åland
Introduction to the geology of southwestern Finland
Olav Eklund1, Sören Fröjdö2, Alexey Shebanov1 & Elin Siggberg2 1Department of Geology, University of Turku, FI-20014, Turku 2Department of Geology and Mineralogy, Åbo Akademi University, FI-20500, Turku
The accretionary arc complex of southern Finland and Russian Karelia
Southwestern Finland belongs to the southern Svecofennian arc complex of the Svecofennian domain. The magmatic history of this arc complex begins with the pre-collisional volcanic-arc magmatism dated to 1.90 Ga. Volcanites with more mature arc signatures formed at 1.88 Ga. The collision of the arc complex towards the central Finland arc complex took place at 1.88–1.86 Ga. These ages are obtained from the syn-collisional calc-alkaline granitoids in the area (Väisänen & Mänttäri, 2002). The syn-collisional granitoids intruded simultaneously with the regional D2. During post-collisional convergence (D3) at 1.84–1.82 Ga, regional high-T and low-P metamorphism was associated with extensive migmatisation and production of melts in the crust, mainly of S-type (Väisänen & Hölttä, 1999). Semi-simultaneously to the metamorphic culmination, the Svecofennian crust of southern Finland and Russian Karelia was intruded by at least 14 small, P-, F-, Ba-, Sr- and LREE-enriched, bimodal, shoshonitic intrusions (Eklund et al., 1998; Väisänen et al., 2000). These small intrusions occur in a 600 km long belt extending from Lake Ladoga in Russian Karelia to the Åland archipelago (Fig. 8). Based on geochemistry, Eklund et al. (1998) concluded that the shoshonitic magmas stem from an enriched lithospheric mantle. Age determinations indicate that this magmatism occurred between 1815 and 1770 Ma, most ages centre ~1800 Ma (Eklund et al., 1998; Väisänen et al., 2000). The oldest shoshonitic rocks were emplaced at mid-crustal levels (~4.5 kbar; Väisänen et al. 2000) and the younger in the upper crust, at approximately 2 kbar and less (Eklund et al., 1998; Eklund & Shebanov, 2005). After the emplacement of the 1.8 Ga post-orogenic intrusions, there was a magmatically quiet period of 100–200 Ma until the voluminous anorogenic A-type rapakivi granites and associated anorthositic rocks and tholeiitic mafic dyke swarms invaded the crust at roughly 1.6 Ga. The 1.58 Ga anorogenic rapakivi granites and associated anorthosites and mafic dyke swarms in SW Finland were emplaced under approx. 1–2 kbar (Shebanov and Eklund, 1997). Consequently, exhumation and uplift between the metamorphic peak and the post-collisional intrusive event must have been fairly rapid, around 3 kbars in 30 Ma based on estimated spread in age from 1815±10 Ma to 1770±2 Ma, compared to almost no exhumation in the following 200 Ma.
Post-orogenic magmatism in southern Finland
At least 14 small (1–11 km across) 1.8 Ga Svecofennian post-collisional bimodal intrusions occur in the accretionary prism of southern Finland extending from the Åland islands to the NW Lake Ladoga region (Fig. 8). The intrusions form a shoshonitic rock series with total alkalis >5%, K2O/Na2O>0.5, Al2O3>9% over a wide range of SiO2 (32–78%) (Fig. 9). The end members of this rock series are ultramafic, calc-alkaline, apatite-rich high-K lamprophyres and peraluminous HiBaSr granites. All rocks in the association are strongly enriched in the LILE Ba and Sr, in the LREE and depleted in the HFSE Ti, Nb and Ta. The trace elements follow a fractionation trend were LILE and LREE are depleted with increasing SiO2. For the lamprophyres and for the granites, the initial Sr values are similar and low (~ 0.7033–0.7047) and the ,Nd values are slightly positive and overlapping (+0.7 to +0.3, Andersson et al in press).
15 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
The Åva ring complex
Geochemistry, petrography and age
The Åva ring complex is one of three bimodal shoshonitic ring complexes situated along a northeast-trending shear zone in soutwestern Finland; Åva, Seglinge and Mosshaga (Fig. 8). The Åva ring complex is approximately 7 km across. The complex comprises hundreds of ring dykes of coarse porphyritic (HiBaSr) granites, monzodiorite, monzonite, quartz-monzonite and granodiorite pillows having shoshonitic geochemical affinity. These more mafic rocks are collectively referred to as “monzonite”. The ring complex is cut by shoshonitic lamprophyres in a radial pattern (Fig. 8). By using the geochemical method developed by Liégeois et al. (1998), it is possible to separate the rocks with alkaline affinity from those whith shoshonitic affinities (Fig. 9b). The former are relatively enriched in Zr, Ce, Sm, Y and Yb while the latter are relatively enriched in Rb, Th, U and Ta compared to a reference rock series. The post-collisional ring complexes in southwestern Finland plot within the field of shoshonitic rocks and may thus, according to Liégeois et al., (1998) be interpreted that they stem from a previously enriched phlogopite- and amphibole-bearing source in the lithospheric mantle.
Figure 8. (a) Geological sketch map and distribution of postcollisional shoshonitic intrusions in southern Finland and Russian Karelia. (b) The Åva ring complex in the Åland archipelago exposed on islands and skerries.
16 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Fig. 9. (a) K2O vs SiO2 plot for postcollisional intrusions in SW Finland showing that almost all these rocks plot in the field for shoshonitic rocks. (b) Trace element diagram separating rocks with shoshonitic affinity from rocks with alkaline affinity. The values of the trace elements are normalized to the Yenchichi-Telabit rock series in the Tuareg shield (NYTS normalization) after Liégeois et al. (1998). The postcollisional rocks follow a well- developed shoshonitic trend with most of the mafic rocks having a prominent shoshonitic affinity, while some of the granites (encircled) are marginally alkaline.
The granite carries K-feldspar megacrysts up to 3 cm in size. The megacrysts are rounded, corroded and, sometimes, subhedral (tabular) in shape. They contain small, fairly rare inclusions of variable amphibole and mica, coexisting plagioclase, sphene and accessory apatite and zircon. Plagioclase occurs as small phenocrysts or as a mantle around the K-feldspar megacrysts. The matrix consists of quartz (often bluish) and biotite, minor plagioclase, K-feldspar and Fe-Ti oxides (mainly titanomagnetite). Occasionally, amphibole is present as inclusions in mica within the matrix of the granite. Both K-feldspar megacrysts and groundmass contain apatite, pyrite, sphene, zircon and fluorite as accessory phases. In places, HiBaSr and sometimes S-type granites also form dykes radial to the ring-complex (Skyttä, 2002). The HiBaSr granite was emplaced semi-simultaneously with palingenic crustal melts, forming a mingled area between these two granites in the central part of the intrusion. Ehlers & Bergman (1984) and Bergman (1986) suggested that the Åva intrusion centres on a gneissic ring structure formed by diapiric emplacement of a late-orogenic S-type microcline granite pluton that deformed the surrounding gneisses into a steep inward dipping ring. The monzonite in Åva was dated at 1799±13 Ma and the granite at 1803±10 Ma (U-Pb in zircon, Suominen, 1991). Field evidence of magma mingling indicates that monzonite and granite magmas intruded contemporaneously. Suominen (1991) also reported younger sphene ages. The reported 207Pb/206Pb sphene ages are between 1754 and 1789 Ma for the monzonites, and between 1759 and 1782 Ma for the granites. Recent SIMS age determinations on zircons situated in different textural positions in the granite evidence a deep mineral assemblage that crystallized at ~1790 Ma and a shallow assemblage that crystallized at ~1760 Ma (Eklund & Shebanov, 2005). In the Åva area, boulders of a rock consisting of plagioclase, orthopyroxene, biotite, amphibole, apatite and minor clinopyroxene have been found. A SIMS age determination shows that rims of zircons have the same age as the monzonite. Mineralogical correlations considering substitution mechanisms in micas and hornblendes imply that the boulders belong to the Åva complex.
17 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Eklund et al. (1998) and Rutanen (2001) have suggested that the HiBaSr granite formed by crystal fractionation of amphibole, biotite, plagioclase, apatite, sphene and magnetite from mafic shoshonitic magma. In this scenario, it is most probable that the boulders represent the high-density cumulus part of a stratified magma chamber deep in the crust. The ring structure is cut by 33 known radial lamprophyre dykes. However, lamprophyres were also emplaced prior to the ring dykes and some lamprophyres were intruded into semi- crystallised granite (Kaitaro, 1953). Petrographically the lamprophyres are spessartites, vogesites and minettes (Hollsten, 1997). Based on geochemistry (Eklund et al., 1998) and mineralogy (Eklund & Shebanov, 2005), it has been concluded that the monzonites principally have the same composition as the lamprophyres; hence they are considered to be plutonic varieties of the lamprophyres.
Petrogenesis
It is suggested that the lithospheric mantle was metasomatised during the subduction stage of the Svecofennian orogeny. An increased thermal gradient melted this phlogopite and amphibole enriched lithospheric mantle at roughly 1.80 Ga, and shoshonitic magmas intruded the juvenile crust and were emplaced at mid-crustal levels. There are some speculations about what caused the melting of the enriched lithospheric mantle. Both Väisänen et al (2000) and Eklund & Shebanov (2002) have suggested that the thermal rise was a consequence of slab breakoff (evidenced by rapid uplift in combination with shoshonitic magmatism). However, since the crustal heating around 1.80 Ga seems to have affected major parts of the Baltic shield, an extensive lithospheric delamination event may have generated the heat. Magma differentiation took place in a mid-crustal magma-chamber (4–7 kbar) about 30 Ma before the time of emplacement of the ring complex in the upper crust (deep assemblage ~1790 Ma, shallow assemblage ~1760 Ma). When about 50% of biotite, amphibole, plagioclase, magnetite, apatite and sphene were fractionated from the initial magmas, peraluminous HiBaSr granites were produced. Crustal assimilation during the fractionation process is considered to be low. The alkali activity was elevated and the environment was relatively reducing in the mid- crustal chamber when compared to the conditions of emplacement in the upper crust. It appears that the juvenile Svecofennian crust was invaded by pulses of shoshonitic magmas from an enriched lithospheric mantle over a long period. Some of these magmas were stored and differentiated in the middle crust before transportation to the upper crust.
STOP 2.1. Ring dykes near Björnholma
The outcrop shows two ring dykes representing the outer part of the ring-intrusion. The dykes are between 40 and 100 cm across and dip steeply towards the centre of the intrusion. They are composed of coarse porphyritic (K-feldspar) granite. The K-feldspars are euhedral, but often mechanically abraded. Towards the contacts, the K-feldspar megacrysts are cumulated.
STOP 2.2. Bimodal lamprophyre–granite dykes at Åva harbour
This outcrop shows a mixture of three components: 1) monzonite, 2) amphibolite fragments, and 3) porphyritic granite. The monzonite forms pillows elongated in the dyke direction in the host porphyritic granite. The amphibolite fragments impinge the monzonite pillows, indicating that the monzonite was plastic. K-feldspar megacrysts from the porphyritic granite are encaptured into the monzonite as well as blue quartz grains forming ocelli texture. These structural and textural features indicate that the monzonitic and granitic magma intruded coevally into the upper crust.
18 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
STOP 2.3. Roof pendants, monzonite-granite interactions, pegmatites
This stop requires a ~2 hour walk during which we will be able to study roof pendants of country rock in the central part of the Åva intrusion. Contact metamorphism has altered calcite to wollastonite in some of the roof pendants. We will also see evidence of an intimate relationship between a monzonitic magma and a cooling granitic magma. Such evidence includes: 1) small mafic and hybrid magmatic enclaves (<5 cm) in in the granite, 2) mafic magmatic enclaves (up to several m) in the granite, 3) lamprophyre dykes which intruded the semi-solid granite, 4) crosscutting lamprophyres, and 5) boulders of deep mafic assemblages. Various vesicles (up to ~1 m with scapolite filling), pegmatites, and post-emplacement mylonites give an insight into the late stage processes in the granite ring intrusion.
Anorogenic magmatism in SW Finland
About 200 Ma after the intrusion of the postcollisional shoshonitic intrusions, anorogenic magmatic activity commenced and the Middle Proterozoic rapakivi complexes and associated diabase dyke swarms were formed. The large ~1.58 Ga rapakivi granite intrusions of Åland, Vehmaa and Laitila, and the smaller Kökar and Fjälskär intrusions dominate the anorogenic rocks in southwestern Finland (Fig. 8). The Åland rapakivi granites are estimated to cover more than 5500 km2 and comprise a great variety of granite types ranging from plutonic to volcanic (Bergman, 1981). The rapakivi massifs are accompanied by about 500 NE trnding tholeiitic dykes in the Åland–Åboland diabase dyke swarm which traverse the archipelago from the Kökar area towards the Vehmaa rapakivi massif. Prevalent in the swarm are narrow and fine-grained hornblende-pyroxene diabases (Ehlers & Ehlers, 1977) of both olivine and quartz normative compositions (Lindberg et al., 1991). Similar diabase dykes are associated with other rapakivi massifs in the Svecofennian domain; e.g., the Häme swarm associated with the Wiborg rapakivi complex in SE Finland (Rämö & Haapala, 1995). In addition to the fine-grained diabase dykes, the anorogenic mafic rocks in the Åland region include leucogabbros and anorthosites (Bergman, 1981). Although the outcrops of these gabbro-anorthosites are small (less than 5 km2 in all), the presence of these rock types in the anorogenic rapakivi-association is important. Age determinations for the felsic anorogenic magmatism in southwestern Finland cluster at about 1575±10 Ma (Suominen, 1991). Field relations evidence the contemporaneity of felsic and mafic activity; e.g. composite dykes, and mingling of mafic and felsic magmas have been described from southwestern Åland (e.g. Eklund, 1993; Lindberg & Eklund, 1988). Fragments of leucogabbro and anorthosite are common in the studied diabases, and plagioclase megacrysts (labradorite) in mafic magmatic enclaves within rapakivi granite varieties of the Åland complex are similar in composition to plagioclase within the gabbro-anorthosites nearby (Fellman, 1994; Lindberg & Eklund, 1992; Shebanov & Eklund, 1997). Bergman (1981) also noted the occurrence of fragments of coarse ophitic diabase (texturally similar to the Korsö high-Al gabbros) within the fine-grained dykes. Commonly the fine-grained diabases and gabbro-anorthosites are considered to be of the same age. However, at all occurrences the gabbro-anorthositic xenoliths are angular fragments, apparently indicating that they were crystalline when incorporated into the diabases, and thus, that the intrusion of fine-grained mafic dykes and rapakivi granites postdates the formation of the anorthositic rocks.
The Korsö dyke.
Of the 500 dykes in the Åland–Åboland dyke swarm Korsö dyke is one of the most extensive and chemically least evolved. The dyke is bimodal (Fig. 10). In its central part it contains a
19 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
phenocryst free A-type granite variety (rapakivi-type?) with miarolitic cavities. Occurence of autoliths of anorthosite and norite (senso stricto) is restricted to the chilled margin and fine grained parts of the dyke. Between the granite and the diabase there is a hybrid zone with sinuous contacts towards the granite as well as to the diabase. The Korsö dyke carries several types of plagioclase cumulates, such as noritic fragments and plagioclase megacrysts, and displays a chilled margin which is one of the most primitive mafic rocks found in the region. The diabase, or leucogabbroic rock, is occasionally very rich in plagioclase, the volume proportion of plagioclase being about 70±5%. The plagioclase is about An65±5, the platy laths are usually ~2.5 cm long. Plagioclase spherulites, indicative of plagioclase over-saturation or supercooled liquids are also found. The observed “stellate” (star-like) plagioclase texture is similar to the “snowflake” textures in the mafic dykes of the Suomenniemi complex, Finland (Rämö, 1991). There is thus ample evidence for the occurrence of high-Al basalts (as noted in the literature) and close association of this texture and massif-type anorthosites—as it is brought forth by plagioclase supersturation in mafic magmas of similar age as the anorthosites.
Polybaric evolution
In a fine-grained part of this dyke a noritic fragment is found in which Al-rich orthopyroxene (Al2O3=4–6%; Mg# ~57) encloses laths of plagioclase (An55). The high Al content indicates crystallization at mid to lower crustal depths, which also is consistent with the lower An content of the plagioclase (Longhi et al., 1993) as compared to the dyke in whole. This indicates that plagioclase was on the liquidus at conciderable depth followed by orthopyroxene. The dyke also contains large euhedral plagioclases (labradorite in composition) up to 7 cm in size. Thermobarometrical investigations on mineral inclusions in these megacrysts indicate a crystallization pressure around 5 kbar, i.e. mid-crustal levels (Shebanov & Eklund 1997). According to phase diagrams developed by Shkodzinsky (1985) magmatic boiling in phenocryst- free A-type granitic melts occurs at pressures less than 0.5 kbar. The presence of miarolites in the granitic part of the dyke indicates that the emplacement level of the dyke was at 0.5 kbar or less. It may thus be concluded that the Korsö dyke presents a record of polybaric evolution through the whole crust.
Figure 10. Schematic map of the bimodal Korsö dyke showing the distribution of various mafic and felsic rock types and autoliths.
20 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
STOP 2.4. Thin tholiitic dykes south of Brändö
This locality enables us to study thin (<1 mm to 30 cm) tholeiitic dykes typical of the Åland– Åboland dyke swarm. The dykes represent low-viscous tholeiitic melts which intruded the country rock and crystallized at a low pressure of <1 kbar.
STOP 2.5. Korsö dyke
The outcrop reveals a major phase of the Korsö dyke. The dyke exhibits a coarse-grained, ophitic texture with plagioclase laths and intercumulus of olivine, clinopyroxene, orthospyroxene and pigeonite. Accumulated plagioclase laths and fragments of leuconorite are found near the contact. The crosscutting dykes are enriched in Ti, P, Fe, and incompatible trace elements.
STOP 2.6. Leuconorite and labradorite megacrysts
Leuconorite fragments with euhedral plagioclase crystals and high-Al opx and labradorite megacryts are found at this outcrop.
STOP 2.7. Vesicular granite, hybrid, and snow-flake plagioclase
At this outcrop we see even-grained A-type granite with vesicles and hybrid varieties between the granite and the Korsö diabase. The diabase contains snowflake plagioclase.
STOP 2.8. Gabbroic autoliths (optional)
Different gabbroic autoliths and large, euhedral plagioclase crystals are found in the Korsö dyke at is outcrop.
The 1583 Ma Vehmaa rapakivi batolith
Five different rapakivi types can be discerned in the Vehmaa batolith: pyterlite (rounded K- feldspar ovoids, rarely having plagioclase rims), two varieties of porphyritic rapakivi granite, even- grained rapakivi granite and porphyritic aplite. They all have different textural, mineralogical and geochemical characteristics. The batolith exhibits a concentric pattern, with pyterlite along the margins and the younger porphyritic granites in the central areas. The porphyritic aplites intruded the pyterlite as minor bodies, except for the semicircular occurrence between two porphyritic rapakivi varieties (STOP 2.10.). Even-grained rapakivi granites (Uhlu granite) form, together with porphyritic aplite, a satellite intrusion east of the main batolith (Linderg & Bergman 1993).
STOP 2.9. Pyterlitic rapakivi granite (optional)
At the harbour of Vousnainen (Kustavi), we can see the uppermost part of the Vehmaa rapakivi batolith. The pyterlite variety contains abundant melt-depleted xenoliths of country rock, aplite dykes with pegmatic pockets, and areas of degassing of the batolith.
STOP 2.10. Järppilä quartz-feldspar porphyry dyke (optional)
This coarse-grained porphyritic aplite dyke intruded between two rapakivi varieties. It contains large (up to 5 cm across) megacrysts of K-feldspar and micaceous clots in a fine-grained (aplitic) matrix. The mineralogy of the clots and mafic silicates inside the K-feldspar megacrysts
21 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
differs from the mineralogy of rapakivi granites in general. The micas have low Fe-index and a high content of Ba (usually the mafic silicates in rapakivi granites have high Fe-index). Mineralogically, the clots thus have a “post-orogenic” rather than an “anorogenic” character. Texturally, the porphyry consists of scattered K-feldspar megacrysts (ovoids), small micaceous clots, and plagioclase megacrysts, scattered in a fine-grained granitic matrix. Geochemically, the porphyries are remarkably enriched in Ba (1900–3700 ppm), Sr (420–780 ppm), and TiO2 (0.68–1.38 %) compared with rapakivi granites in the batolith. These “primitive” features disconcert with late intrusive nature of the Järppilä dyke and are caused by the rather unusual composition of the abundant mica. The studied biotites are characterized by brittle mica substitions (Ba, Ca, Sr) operative in interlayer site, apparent deficiency in tetrahedral site, and relatively low Fe/(Fe + Mg)-ratio (average 0.69). Substitution of Ti 4+ in the tetrahedral site of the most Ba-rich micas is suggested to maintain charge balance. These characteristics differ markedly from the well-restricted compositional space of rapakivi biotites elsewhere. However, Järppilä biotites are nearly identical to barian biotites recognised in the postorogenic (1.77–1.80 Ga) bimodal granite-lamprophyre complexes with shoshonitic affinity, which are often spatially related to the rapakivi batholiths in the area. This fact suggests a relic origin for barian-titanian biotite-bearing assemblages, pointing to postorogenic suite as suitable protolith for the Järppilä porphyries. Aimed to verify that, U-Pb dating by using IMS-facility (Nordsim) has been undertaken. Two distinguishable populations of zircons have been investigated: a) nearly stoichiometric, Hf-poor zircons found mainly as inclusions within K-feldspar ovoids and b) zircons exhibiting elevated Zr/Hf and peculiar disturbances in stoichiometry; these zircons predominately occur in the matrix. The first population sometimes contains inclusions of Ba-enriched minerals and yields a discordia with an upper intercept of 1616±7 Ma, MSWD = 1.2. The second group does not assemble with barian mica-bearing paragenesis and yields an emplacement age as defined by a separate discordia with an upper intercept of 1579±6 Ma, MSWD = 0.47. These two events seem to be reflected in the clustering of different mineral assemblages in their isotope compositions. Rb-Sr isotope data for mineral inclusions within the ovoids plot on a reference line with an age of 1604±30 Ma and initial 87Sr/86Sr= 0.7018±0.0004, whereas the rest forms a separate errorchrone defining an age of 1577±19 Ma, initial 87Sr/86Sr = 0.7040±0.0001 (MSWD = 1.26). The former value indicates a presence of the mantle-derived component for the first assemblage encapsulated within ovoids. This accords with significantly more positive time integrated ,Nd(1.58 Ga)= +2.0 for the first assemblage compared to the second assemblage that yielded ,Nd(1.58 Ga)= –1 0 to – 4.0. The latter is equal to common bulk-probe characteristics for Finnish rapakivi granites. In sum, data on the relic mineral assemblage in the Järppilä rapakivi porphyries favour a mantle-derived initial source (late postorogenic?) which experienced short residence in the crust not long before the anorogenic 1.58 magmatic event. Our data suggest that postorogenic magmatic activity might have continued up to approximately 1.62 Ga.
22 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Day 3: The Subjotnian Häme swarm
Arto Luttinen Department of Geology, P.O.Box 64, FI-00014 University of Helsinki, Finland
The following description of the Häme swarm and the excursion stops is largely based on the PhD thesis and other published work of Ilkka Laitakari.
STOP 3.1. Proterozoic basaltic glass in Orivesi dykes
In the Häme dyke swarm, undevitrified Precambrian glass has been found (Lindqvist & Laitakari, 1980). It appears in the narrow (<2 cm) tails of ~10 m long and ~10 cm wide diabase apophyses. The parent diabase is 2.7 m wide, vertical, and strikes NNW. The glass is found ~25 m from the dyke in an apophysis striking NNE and dipping 60 ESE in a road cut near Aihtianjärvi, Orivesi. The apophyses cut a Svecofennian hypabyssal granitoid. The fresh broken surface of the glass is lustrous black, but in thin section it is brown. Altered parts are greenish blue and slightly birefringent. Traces of devitrification can be seen along tiny cracks and at the contacts. The apophyse tails are hyalophitic and the glass content is up to 90%. Two generations of plagioclase (An55–60) and sporadic olivine are found as phenocryst phases in the samples studied by Lindqvist & Laitakari (1980), elsewhere, there are also larger, up to ~2 cm long plagioclase macrocrysts. The large ~1 mm plagioclase phenocrysts are equidimensional, whereas the smaller ones are ~0.3 mm long laths. The laths show flow orientation. Compositionally distinct, yellow domains are found within the brown glass. These patches are relatively poorer in SiO2, CaO and Na2O and higher in MgO and FeOtot compared with the enclosing brown glass. Euhedral, partly serpentinized olivine has been found in the yellow patches (Fig. 11). The origin of the yellow patches has been ascribed to immiscibility between two silicate liquids by Lindqvist & Laitakari (1980). Geochemically, the glass is a subalkaline, tholeiitic basalt and quite similar to the assumed parent diabase dyke. The composition is typical of an evolved tholeiite with low MgO (4.2 %), Ni (45 ppm) and Cr (46 ppm) combined with relatively high FeOtot (13.5 %), TiO2 (2.8 %), and incompatible trace elements, such as Zr (230 ppm). The notably high water content of the glass (8.8 %) is believed to be of magmatic origin and may have fascilitated the formation of a glass apophysis by lowering the viscocity of the basaltic melt. In the same area (at Leväslahti), an amygdaloidal diabase has been found. This dyke is 8 m wide, strikes N85W and dips 70N, and has a subophitic texture. The amygdules are spheroidal, about 2 mm in diameter, and consist of calcite and euhedral quartz. There is a 40 cm wide amygdule-free zone below the upper contact. The presence of amygdules has been regarded to indicate that the present erosion level is close to that of ~1600 Ma ago.
Sampling is NOT allowed on this outcrop! The road has heavy traffic -watch out!
STOP 3.2. Flow-differentiated Vehkajärvi dyke
The diabase dyke of Vehkajärvi is exceptionally well-exposed in a road cutting and, accordingly, has been fairly intensively studied in the past (Laitakari, 1969; Boyd, 1972; Mäkipää, 1979). The 3 m wide dyke has sharp contacts with the wall-rock greywacke schist. It is black, fine- grained and has glassy margins with small, needle-like plagioclase laths. There is a 50 cm wide zone in the centre of the dyke which is markedly rich in up to 10 cm long megacrysts. The
23 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
plagioclase megacrysts have cores of An58, enclosed in a narrow rim of An62 and an outer rim of An52 (Boyd, 1972). Geochemical traverses of the Vehkajärvi dyke have been reported by Boyd (1972) and Mäkipää (1979). The data for 13 whole-rock and 109 groundmass analyses indicate a uniform subalkaline basaltic composition across the dyke. High concentration of plagioclase megacrysts in the centre of the dyke is reflected by the increased Al2O3 (~19 %) and CaO (~8.6 %) contents in the whole-rock data (Fig. 12), while the groundmass data indicate a slight decrease in these components. The groundmass in the Vehkajärvi dyke is quartz-tholeiitic and fairly low in MgO (3.3 %) and high in TiO2 (2.6 %), P2O5 (~0.8 %), and Zr (~340 ppm). It is actually one of the most evolved diabase dykes so far analysed in the Häme swarm. The major element composition corresponds to those of basalt liquids that are in equilibrium with olivine, plagioclase, and clinopyroxene at one atmosphere pressure (Boyd, 1972). According to Mäkipää (1979), the large megacrysts have not crystallized from a melt corresponding to the composition of the surrounding matrix.
Sampling is only allowed with permission from the excursion leader!
STOP 3.3. Quartzite cobbles in Maijaanvuori dyke (optional)
Phenocrysts, megacrysts and large fragments of plagioclase (An55–60) are common in the Häme swarm; they have been observed in 83 of the 109 studied dykes. The occurrence of phenocrysts, megacrysts, and fragments is limited to the younger 1646±6 Ma set of dykes, however (Laitakari & Leino, 1989). In 39 dykes at least some of them exceed 5 cm in diameter; the largest crystals attain ~30 cm in length (e.g. Maijaanvuori) and typically are almost euhedral. In 19 dykes there are autoliths. They are usually <30 cm in diameter. Some of them resemble coarse-grained diabase or diabase pegmatite and others are anorthositic. Plagioclase (An48–53) is the predominant mineral and interstitial hypersthene is the main mafic mineral. One dunitic inclusion has also been discovered. Judging from the presence of interstitial plagioclase (An56), it is probably a low- pressure cumulate and does not have a mantle origin. The similarities between the autoliths and megacrysts of the dykes to the rocks of the Ahvenisto gabbro–anorthosite complex and the fact that dykes become narrower towards the northwest have led to the interpretation that the transport of magma was essentially lateral and that the Ahvenisto pluton may well represent the magma source of the Häme dykes (Laitakari & Leino, 1989). Many of these dykes (n=12) also contain quartzite xenoliths which can comprise up to ~20 % of the diabase (Fig. 11). Quartzite-bearing dykes are found over ~90 km long area west of Lake Päijänne. The xenoliths are usually angular, but the Maijaanvuori dyke contains completely rounded xenoliths. The inclusions are typically 2–10 cm in diameter, but the largests exceed 20 cm. Two varieties of quartzite have been identified: Gray ones are most common, but the white type includes most of the large xenoliths. The gray xenoliths have a blastoclastic texture and have minor sericite, biotite, plagioclase, opaque minerals, apatite, and zircon. The grain size is ~0.1–0.2 mm. The source of the quartzite xenoliths is unknown; no quartzite of this type is known in the area. The minor quartzite occurrences are clearly schistose and more deformed than the xenoliths. Two obvious solutions that have been considered by Laitakari and his co-workers are: derivation of quartzite from sources 1) below or 2) above the present erosion level. Laitakari & Leino (1988) envisaged that many of the Häme diabase dykes reached the surface in Proterozoic time, that “magma ran in the fissures like water in a river”, and that the quartzite xenoliths fell into the lava stream. In this scenario, the dykes may well have acted as feeder systems of fissure eruptions not unlike the Laki eruption of Iceland in 1783–1785.
24 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 11. Photomicrograph of glassy Orivesi dyke showing brown glass (b), plagioclase (pl), and a partly serpentinized euhedral olivine (ol) in yellow glass (y) (LEFT). Quartzite xenoliths in Jouttijärvi diabase (RIGHT).
Figure 12. Al2O3, CaO and MgO variation and the variation of the grain size of the Vehkajärvi dyke. The numbers below the photos give distance from the contact (Mäkipää, 1981).
Be ready for mosquitos at this outcrop!
25 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Day 4: Microtonalite dykes, kimberlites, and soapstone factory
Intermediate magmatism at the craton border zone, central Finland
Olli Äikäs1 & Arto Luttinen2 1Geological Survey of Finland, P.O.Box 1237, FIN-70211 Kuopio 2Department of Geology, P.O.Box 64, FIN-00014 University of Helsinki
The western border zone of the Archaean bedrock complex with the overlying Early Proterozoic supracrustal rocks are cut by numerous intrusions ranging from felsic to mafic in their chemical compositions. Intermediate magmatic rocks make up one distinct group among these intrusions, predominantly occurring in a relatively narrow zone from the line Leppävirta– Tuusniemi–Kaavi up to the area west of Lake Oulujärvi. Tentatively named as microtonalites (Huhma, 1981), these rocks comprise a texturally, structurally and compositionally varying series of intermediate rocks, which occur mainly as dykes. Dating of this intermediate magmatism has been problematic, mainly due to contamination. According to Huhma (1981), the microtonalite dykes in the Kaavi area postdate the 1.86 Ga Maarianvaara granite intrusion, but predate the 1.83 Ga lamprophyre dykes of the area. A U-Pb sphene age of 1.857 Ga for a sample from Kaavi (A940, Fig. 13) has been considered to give a minimum age for the microtonalite dykes. Another dyke from Murtolahti, Nilsiä yields ages of 1829±13 Ma for sphene, and 1835–1939 Ma for a heterogeneous population of zircon (Irmeli Mänttäri, personal communication, 2000). Microtonalite dykes are numerous also in the Syväri region in Nilsiä, between Tahkomäki and Lastukoski, where the Archaean basement is dominated by paragneisses (Paavola, 1984). On the basis of field observations mainly in the Juankoski map sheet area (Äikäs, 2000), several pulses exist, although the emplacement ages may be indistinguishable using radiometric dating. The dykes can be grouped in the following fashion: 1) Conformable or semiconformable dyke segments in the Archaean bedrock; 2) Dykes that crosscut Early Proterozoic mica gneiss but have been deformed and broken apart into boudins during the early stages of the Svecofennian orogeny; 3) Dykes that crosscut the Archaean bedrock, Proterozoic quartzite, metadiabases, and the 1869±5 Ma (Hannu Huhma, personal communication, 2005) Juurus tonalite; and 4) Dykes that are associated with NW-trending granite pegmatite dykes and, in places, crosscut other microtonalite dykes. According to Rautiainen (2000), microtonalite dykes sampled at the craton border zone are fine-grained, massive, light to dark gray, and homogeneous. They have sharp, often jagged contacts with the wall-rock; in many places, the tonalite dykes brecciate the wall-rock. Apophyses are common. The dykes are 4–5 m wide and strike NE-E. Many show weak magmatic banding. The dykes have a hypidiomorphic texture and consist mainly of subhedral plagioclase, quartz, biotite, and hornblende. Epidote, sphene, apatite, and opaque minerals are accessory mineral phases. The dykes can be classified as tonalite and subordinate quartz diorite; some plot in diorite/gabbro field, but they are collectively referred to as microtonalite. Plagioclase phenocrysts in the microtonalite dykes are 0.5–2 mm long and show zoning from An49 to An15. The roundness and the resorbed appearance of phenocrysts, needle-like apatite, crystal aggregates, micrographic texture, and quartz-plagioclase ocelli are characteristic indicators of mixing processes. The intermediate dykes include bimodal dykes with mafic enclaves within felsic “tonalite” (Fig. 14). Phenocrysts in the mafic enclaves and in the felsic parts are compositionally similar. The enclaves range from basalt to basaltic andesite with SiO2 of 43–55 %, whereas the felsic host rock is andesite to rhyolite with SiO2 of 61–72 %. Compositions of the homogenous dykes are intermediate between the mafic and felsic end-members. Overall, the enclaves, the felsic end- member, and the homogeneous dykes define linear arrays in geochemical diagrams: CaO, FeOtot, MgO, TiO2, MnO, and P2O5 decrease (9–4 %, 14–4 %, 6–2 %, 2–0.6 %, 0.23–0.05%, 1–0.2 %,
26 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
respectively) when SiO2 increases from 43 to 64 %. Combined with petrographic evidence of magma mixing and mingling, the linear trends indicate that the microtonalite suite was generated as a consequence of interaction between a basaltic magma and a felsic magma. High La/Nb (1.6–3.2) and Ba/La (18–20) of the most mafic samples (SiO2 51–48 %) are typical of subduction-related magmas. In AFM-diagram, these samples plot within the calc-alkaline field.
Figure 13. Simplified geological map of the western North Karelia from Huhma (1981): 1 – Archaean rocks, 2 – Early Proterozoic rocks, 3 – microtonalite dykes, 4 – lamprophyre dykes. In the upper left corner elements from the geological map sheet area 3333 Juankoski (Äikäs, 2000) have been added: grey – Archaean; white – Early Proterozoic; crosses and spots in black – microtonalite outcrops; symbols in magenta – kimberlite; red dot – location of the Kivennapa field stops. The symbol for kimberlite NE of Kaavi denotes the Lahtojoki pipes and the Kaavi kimberlite cluster in general.
27 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 14. Pillow-like mafic enclaves in felsic material from Honkamäki, Nilsiä, 10 km SW of Kivennapa (Rautiainen, 2000). The length of the tag is 10 cm. Photo: J. Rautiainen.
Figure 15. Typical composite dyke of microtonalite intruding the Juurus tonalite in the "Saunaranta" outcrop at Kivennapa, Juankoski. Photo: O. Äikäs.
STOP 4.1. Microtonalite dykes at Kivennapa
The bedrock exposures at Kivennapa provide an insight into the complex setting of the microtonalite dykes (Figs. 15 and 16). Composite dykes with leucotonalitic seams between the mafic parts intrude and brecciate the Juurus tonalite. Later dykes of the leucotonalitic material crosscut the darker microtonalite. More complex outcrops show lithologies from the migmatitic and folded and boudinaged Early Proterozoic mica schist to late granite pegmatite with successive intrusive phases of Juurus tonalite and dykes of microtonalite, granodiorite, and granite. In addition, a large erratic boulder of microtonalite presents an eaxample of the porphyritic texture typical of numerous dykes in the Juankoski map sheet area.
28 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 16. Detailed map of the "Porch" outcrop at Kivennapa, Juankoski. Map by O. Äikäs.
STOP 4.2. The Muuruvesi church
Various small-scale features, such as interaction between mafic and felsic magma, can be readily examined on the stone walls of the Muuruvesi church (Fig. 17), 12 km SE of Kivennapa.
Figure 17. Blocks of microtonalite (from composite dyke; below) and Juurus tonalite (above) in the wall of the Muuruvesi church. Photo: O. Äikäs.
29 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Eastern Finland Kimberlite Province
Hugh O´Brien1 & Matti Tyni2 1Geological Survey of Finland, P.O.Box 96, FIN-02151 Espoo 2Korvenkuja 7, 62310 Voltti
Intrusion morphology and Facies
The kimberlites from northeastern Finland range from <1 hectare to almost 4 hectares. The intrusions range from circular to elliptical pipes to dikes up to 0.5 km in length (Fig. 18). Surface exposure of the pipes is poor, however, Pipes 1 and 2 are presently exposed due to recent exploration and diamond evaluation activities. Tuffisitic diatreme facies rocks are dominant, although several of the intrusions are composed entirely of hypabyssal material (Table 1). Evidence for possible crater facies material is limited to pipe 5 which contains volcaniclastic rocks that vary from breccias to ash-rich sandstones. It is unclear whether the sandstones represent downrafted blocks of crater facies material or simply more completely comminuted diatreme material.
Kimberlite samples
Pipe 1, the "strange rock" discovered in 1964, is the only pipe that does not contain breccia, is almost xenolith-free, and represents the least contaminated kimberlite in this suite of samples. As it is also by far the least altered, it makes an ideal endmember with which to compare the remaining samples. Containing up to 40% olivine as macrocrysts and phenocrysts in a matrix of monticellite microphenocrysts, perovskite, chromite zoned to magnesian ulvöspinel-magnetite (MUM), calcite, serpentine, and kinoshitalite mica, the main phase of the pipe is mostly massive but about 10% has a carbonate segregation-texture. The marginal phase of the pipe is quite distinct and is formed of autoliths of the kimberlite magma cemented into a carbonate matrix (25% by volume). These autoliths contain abundant spheres of calcite that likely record the separation of an immiscible carbonate magma that coalesced near the margin of the pipe to form the carbonate matrix. The freshest samples of the other nine kimberlites have the same matrix minerals as those in Pipe 1, but in most cases only pseudomorphs of monticellite remain, the bulk of olivine has been serpentinized and there is more sphene as opposed to MUM. An additional important component in the other kimberlites is varying amounts of xenoliths (described below), xenocrystal peridotite detritus (esp. olivine, chromite, chrome pyrope, chrome diopside), megacryst suite garnet, ilmenite, pyroxene and abundant crustal detritus in the diatreme breccias. Some parts of pipes 5 and 14 in particular are crystallinoclastic, comprising essentially disaggregated peridotite minerals in minimal kimberlite matrix.
Xenoliths
Ultramafic to mafic mantle xenoliths are relatively common in pipes 5, 9, 10 and 14 and comprise a suite ranging from garnet websterite to lherzolite, harzburgite and eclogite. Pipe 10 contains a rich variety of eclogites, phlogopite-bearing peridotites and Cr-rich to Cr-poor lherzolites. Abundant xenoliths of lower and upper crustal wall-rocks range from granites to sulfide-rich black schists, quartzites and two pyroxene granulites. Typically the largest population in any given pipe represents the highest level wall-rocks encountered, but in all cases xenoliths of felsic gneisses derived from Archean gneiss basement occur. Spectacular tuffisitic breccias from pipe 3 contain abundant xenoliths of quartzite, gneisses and granitoids. However, the xenolith population of the tuffisitic rocks varies dramatically among drill cores located only a few tens of meters apart, and the quartzite xenoliths do not occur in any other drill core intercepts.
30 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 18. Surface plan maps of kimberlite pipes from Kaavi (LEFT) and distribution of kimberlites in the Kaavi region (RIGHT).
Mineral chemistry
Olivine–Fresh olivine exists only in pipes 1, 5, 9 and 14. The forsterite-rich olivines from pipes 1 and 14 have a restricted compositional range, from Fo87 to Fo92 which includes olivine from peridotite xenoliths, rounded macrocrysts and smaller euhedral to subhedral phenocrysts that presumably crystallized from the kimberlite magmas. The larger olivine grains are typically unzoned but it is common for the smaller phenocrysts to show core Fo90 to rim Fo88 variations. Olivines from pipe 5 and 9 appear to have a bimodal compositional distribution with a Fo93–89 population like those described above and a Fo86–83 population representing megacryst-suite olivine. Monticellite–Monticellite is common in Pipe 1, rare in Pipe 14 and found only as inclusions in mica and sphene in pipe 10. It is absent from the remaining pipes which are all in less pristine condition. Euhedral monticellite grains, mostly from 10 to 50 micrometers in size, form the main component of the groundmass in the Pipe 1 samples. Those that occur in Pipes 10 and 14 are partially altered microphenocrysts, quite similar in size and composition to those in Pipe 1. Mica–Ba-rich mica (kinoshitalite) occurs in the groundmass of virtually all of the Finnish kimberlite samples. Its abundance ranges from very sparse as in Pipe 9, to as much as 10% of the matrix, as in Pipe 10. The Finnish examples range to extremely Ba-rich compositions (up to 17.8% BaO), especially those from Pipe 1, and also contain a large amount of fluorine. Spinel–Pipe 1 samples contain up to 5% spinel, which uniformly have spectacular atoll structures. In thin section translucent dark red chromite cores are surrounded by an amorphous serpentine-like material which is in turn mantled by magnesian ulvöspinel and surrounded by an outer thin rim of magnetite (Fig. 19a). However, in rare cases it is possible to find remnant pleonaste spinel that originally filled the "lagoon" in all of these atoll spinel grains (Fig. 19b). In detail the chromite cores are typical kimberlite spinel magmatic trend 1 (Pasteris, 1983, Mitchell, 1986) titanian aluminous magnesian chromite (TIMAC) but are abruptly zoned to titanium-bearing, nearly chrome-free pleonaste spinel, quite away from the normal zoning in Trend 1 spinels (op.cit.). The MUM mantles are practically unzoned and a few, in addition to an outer rim of magnetite, have an inner, discontinuous magnetite rim of the same composition (see Fig. 19b). Pipes 2, 3 and 5 also contain atoll spinels analogous to those described above although they are less
31 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
abundant and suffer from the relatively altered condition of the host rocks. Somewhat similar atoll spinels with considerably thinner pleonaste rims and titanomagnetite cores instead of chromite were described from the De Beers kimberlite (Pasteris, 1983). Additional information on the growth of the pleonaste spinels comes from mantles developed on large olivine macrocrysts and rounded olivine-monticellite-spinel cumulate fragments that form what we have termed "orbicules" in several Pipe 1 samples. In many respects these are similar to the "spheroids" described by Reid et al. (1975) from the Igwisi Hills. Ilmenite–Magnesium-rich ilmenite (picroilmenite), a typical mineral found in kimberlites and representing one of the most important diamond indicator minerals, is abundant in pipes 4, 5, 6, 9, 10, 14 and 23. In a similar fashion to the spinels, ilmenite compositional zoning patterns in the Finnish kimberlites are complex. Thus far three main types of zoning have been identified. a. Decreasing Cr with increasing Mg toward rim. High Cr (ca. 5 wt% Cr2O3) picroilmenite macrocrysts from Pipe 14 have homogeneous cores, but show decreasing Cr within 20 microns of the crystal edge (Fig 20a). The lowest Cr, highest Mg picroilmenites occur just inside the very edge of the crystals, adjacent to a few micrometer wide, outer MUM rim. b. Increasing Cr and Mg toward rim.This is the predominant type of zoning developed in picroilmenite macrocrysts with low Cr cores. Well documented in kimberlite picroilmenites (Mitchell, 1986), it is manifested as diffuse zoning across most of the crystal at a relatively constant Cr content and from 10 to 12 % MgO. However, near the crystal edge a much more abrupt zoning pattern is developed, with a large increase in MgO and Cr2O3 over distances of tens of micrometers. At the very edge of the crystal a ca. 5 mm reaction rim of MUM spinel occurs on these grains. The high Cr ilmenite reaction zone mimics the shape of the MUM rim and where there are embayments of MUM farther into the ilmenite crystal, the high Cr picroilmenite reaction zone rather uniformly outlines the embayment. However, in some cases the core composition occurs right up to the present crystal edge, which probably reflects fracturing of the crystal shortly before pipe emplacement. In the same sample, another version of zoning with an increase in Cr and Mg toward the rim occurs in picroilmenites from peridotite microxenoliths. The microxenolith consists of a large Fo89 olivine, a very magnesian (~14% MgO) ilmenite and between them a patch of chalcopyrite and associated djerfisherite. Olivine included in the picroilmenite is also Fo89, and this, coupled with the rounded shape of the microxenolith, indicates that the fragment represents a mantle sample. Developed only at the outer edge of the microxenolith, where the ilmenite was exposed to kimberlite magma, is distinct zoning to more MgO-rich compositions and a sharply defined MUM outer rim. There is no zoning in the ilmenite toward the area of serpentinized olivine, indicating that this late stage process had no effect on ilmenite compositions. Chrome and Mg-rich picroilmenite inclusions are relatively common in olivine macrocrysts in this sample and have very similar compositions to the microxenolith ilmenite. There is no difference between those inclusions that occur in fresh olivine and those that occur in serpentinized olivine further demonstrating that late stage serpentinizing fluids had no affect on the ilmenites and in particular did not contribute to the development of the spinel reaction rims. c. Increase in Mg, no increase in Cr toward rim. A completely different zoning trend from b. is developed in many low Cr picroilmenite cores and derived fragments; one of increasing Mg at nearly constant Cr toward the grain rim. The example shown in Fig. 20b has a relatively wide reaction zone and a correspondingly wide rim of MUM spinel. Remnant patches of ilmenite within the MUM rim attest to the formation of the latter by the reaction ilmenite + liquid = MUM. Supporting this is the fact that the spinel rim compositions reflect their ilmenite precursors systematically, i.e., the highest Cr ilmenite has the highest Cr spinel reaction rim and so on. Distinct from the ilmenites described above are the extremely Cr-rich examples from Pipe 9. These contain up to 12 wt% Cr2O3 and consist of an ilmenite host with submicron Cr-rich planar domains that appear to be chromite exsolution lamellae.
31 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Apatite and Perovskite–Abundant apatite occurs as acicular grains commonly in radiating stellate clusters in the groundmass and as larger more prismatic grains grown primarily within calcite segregations. They are relatively Si-rich (0.7–1.1% SiO2) and Sr-poor (<1% SrO), characteristic of Group I kimberlite apatite. The majority of the perovskite occurs as euhedral to subhedral discrete grains (rarely as aggregates) that are 0.02 to 0.1 mm across. Our limited data on perovskite shows them also to be typical of Group I kimberlites with ~1.3% FeO, ~1.5% Nb2O5 and 0.1–0.3% SrO.
Figure 19. (a) Typical complexly zoned spinel from Pipe 1. Core of grainis titanian aluminous chromite zoned to chrome spinel with a magnesian ulvöspinel magnetite mantle and a very outer rim of titanian magnetite. (b) Rare example of pleonaste growth layer still partly intact.
Figure 20. (a) Example of high Cr picroilmenite core zoned to a lower Cr rim. (b) Small fragment of a low Cr, low Mg picroilmenite core that is zoned to higher Mg (dark orange to purple in this BSE image) without an increase in Cr. Relatively wide spinel reaction zone retains some remnants of the precursor ilmenite (notably in the lower left). Geochemistry
32 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Major elements of the Finnish kimberlites show that the samples vary from being unaltered and uncontaminated (e.g., Pipe 1) to highly altered and/or contaminated (e.g., tuffisitic diatreme samples from Pipes 4 & 6). Only a few of the samples are uncontaminated or slightly contaminated; the geochemical arrays point in the direction of increasing contamination for Pipes 3 and 14. Not unexpectedly, granitoid material appears to be the most significant contaminant. Despite the indication for contamination from major elements, incompatible trace elements show that this contamination mostly had a dilutive effect. Three samples from Pipe 1 have nearly identical trace element compositions and represent the closest example to an uncontaminated kimberlite. Relative to this Pipe 1 composition, the more contaminated samples have lower concentrations of nearly all of the incompatible trace elements except K and Rb (Fig. 21). Importantly, there is no evidence that the shape of the trace element profile was changed significantly by the assimilation process. This is probably simply a consequence of the very high trace element concentrations in the original kimberlite magma relative to the crustal assimilant. Compared to average Group I kimberlites, the Finnish examples have very similar incompatible element concentrations (Fig. 21). The only systematic differences are the considerably lower concentrations of Hf and Zr and higher Ba in the latter. This is shown clearly in a plot of Zr-Nb (Fig. 22) which compares kimberlites from various localities around the world. The Finnish kimberlites plot with the relatively low Zr Koidu kimberlites of W. Africa. Initial 87Sr/86Sr isotopic compositions of the Finnish kimberlites range from 0.7033 to 0.7055 calculated for ages of 434 Ma (Pipe 1), 593 Ma (Pipes 2 &3) and 450 Ma for the remainder. Initial ,Nd values range from +1 to –2.4. The Sr compositions overlap with and extend to more radiogenic values than Group I kimberlites, but the ,Nd values, centered on bulk earth composition, are slightly lower (Fig. 23). It must be noted that the Finnish kimberlites are not yet well dated (only two K-Ar ages) leaving open the possibility that this field may shift slightly once more age dating has been completed.
Figure 21. Primitive mantle-normalized incompatible element diagram showing selected Finnish kimberlites and average Group IA and IB kimberlites (values from Smith et al., 1985 and Mitchell, 1986; primitive mantle from Sun & McDonough, 1989).
33 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 22. Nb vs. Zr for the Finnish kimberlites and related rocks which show that the least contaminated kimbelirte compositions plot in the Koidu kimberlite field (Taylor et al., 1994) and have lower Zr than the Group I kimberlites from South Africa. Also shown is lamproite- like Dyke 16.
Figure 23. Sr vs. Nd isotopic diagram for Finnish kimberlites. Note overlap with the Koidu kimberlites.
34 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Discussion and conclusions
A basic tenet of kimberlite petrology is that kimberlites are very complex and represent mixes of various batches of magma and their phenocrysts along with abundant mantle xenocrysts (Mitchell, 1986). They therefore do not represent true magmatic liquids. The kimberlites from Finland exhibit typical mineralogy of Group I kimberlites from elsewhere in the world including abundant macrocrysts of olivine, picroilmenite, Cr-diopside, pyrope garnet; phenocrysts of olivine and microphenocrysts of monticellite, perovskite and spinel in a calcite + serpentine matrix. Notably phlogopite phenocrysts are not abundant, and there are no indications of Group II kimberlite (orangeite) affinities in these rocks. Detailed studies of the complex zoning in constituent minerals reveal wide variations in the compositions of the magmas that mixed to form the kimberlites discussed here. Major elements indicate that there is a crustal component in some of the samples although trace elements show that this assimilation has had mostly a dilutive effect. The Finnish kimberlites form a compositionally cohesive group of magmas more akin geochemically to the Koidu type kimberlites of West Africa than the Group I kimberlites from South Africa. Sr-Nd isotopic compositions either reflect derivation from a near bulk-earth mantle or represent mixing of Group I kimberlite isotopic sources and a more radiogenic Karelian Craton lithospheric mantle.
Figure 24. Topographic map showing the distribution of kimberlite boulders (stop 4.2.) and Pipes 2 (stop 4.3.) and 3.
35 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
STOP 4.2. Kimberlite boulders
Late in the 1970´s, glacial boulders of well-preserved “almond rocks” were found. The name referred to the numerous white almond-shaped inclusions that were later proven to be altered peridotite xenoliths. Samples of these strange rocks were sent to diamond companies where they were correctly identified and found to contain microdiamonds.
STOP 4.3. Pipe 2 Mäntyjärvi Niilonsuo
Pipe 2 was located by Malmikaivos Oy under a small swamp, about 1 km up-ice from the glacial boulders (Fig. 24). Pipe 2 is an elongate body, roughly 300 m in length (Fig. 18). It contains 593 Ma hypabyssal kimberlite and volcaniclastic breccias. The hypabyssal rocks contain serpentine pseudomorphs after olivine macrocrysts in a matrix of perovskite, serpentine, and calcite.
STOP 4.4. Soapstone factory (Tulikivi)
This combined lunch-break and excursion stop gives us an opportunity to learn about the history and present activities of the Tulikivi Corporation at Juuka. Tulikivi Corporation and its subsidiaries form the Tulikivi Group, which is the world’s largest and most technologically advanced soapstone processing company as well as the largest manufacturer of industrially produced heat-retaining fireplaces in the world. The Group is among the five largest stone processing companies in Europe. In 2004, the Group’s revenue amounted to MEUR 55.3. The Group has six production facilities, and its personnel consists of approximately 515 employees. The Tulikivi Group focuses on stone processing as well as on the development and customer- oriented manufacture of natural stone products and services. Tulikivi’s Fireplace Business comprises soapstone quarrying, production, design, sales and marketing. The products include soapstone fireplaces, bakeovens, cook stoves, custom-made products and sauna stoves. In addition, soapstone lining stones to European heater manufacturers are supplied. Tulikivi Corporation’s Architectural Stone Business comprises household interior decoration stone products and deliveries of stone materials for construction sites. The products are made of soapstone, granite, marble, limestone and other natural stones. The interior decoration stone products are suitable for various household uses, such as for the kitchen, the bathroom, floors, stairs and wall tiling – and as architectural stones, too. Soapstone's reputation as a "wonder stone" first reached Helsinki in 1893, when a group of distinguished industrialists founded the Finnish Soapstone Company. Soapstone's fame spread rapidly to the imperial courts of St. Petersburg and Moscow. The stone was shipped from Lake Pielinen via the Saimaa Canal to Russia to become building and fireplace stone material for the palaces. A village formed around the quarry. Hundreds of stone workers moved into the area along with their families. The Art Nouveau period was the golden age of soapstone. In the hands of skilled sculptors, soapstone yielded to splendid columns, reliefs, facade decorations and stairways in important buildings in Finland, still visible today. With the Russian revolution the export ceased. After the Second World War the quarrying activity in Nunnanlahti came to a standstill. The village was left deserted for twenty years. In the summer of 1979 all that was left in Nunnanlahti was an abandoned quarry surrounded by a thick row of shrubs. In 1980 the old enterprise reopened its operation, this time under the guidance of new entrepreneurs, Eliisa and Reijo Vauhkonen. The company soon began to use the name Tulikivi, which means "fire stone" in Finnish.
36 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Day 5: Koli layered sill, 1.97 Ga dykes, Stone centre, and Varpaisjärvi dykes
The Koli layered sill in North Karelia, Eastern Finland
Jouni Vuollo Geological Survey of Finland, P.O. Box 77, FIN-96101, Rovaniemi
Introduction
The Proterozoic quartzite hill of Koli is a famous tourist centre which offers beautiful views over the Archaean bedrock terrain to the northeast and the Proterozoic domain to the southwest. We also propose that the name of the hill should be applied to our present topic – the peculiar layered sill which runs NW–SE, passes the hill of Koli beneath Lake Pielinen, and reaches the land at Kaunisniemi, where it is very clearly visible. The sill has been under investigation by members of the University of Oulu (Piirainen, 1969; Honkamo, 1972; Hanski, 1982, 1984, 1986; Vuollo, 1988; Vuollo & Piirainen, 1992) for a long time as a part of a theme "Basic igneous activities and related ore deposits in Finland", being an example of the Jatulian magmatism of age about 2.2 Ga. The sill is peculiar in many respects. It has Al203/TiO2 and Ti/V ratios of 5–6 and 31, respectively, Fe and LREE are enriched and Al depleted, and the Mg' value is 0.59. Its cumulus minerals are olivine, (chromite), clinopyroxene, ilmenomagnetite, (apatite), plagioclase and intercumulus mineral hornblende, clinopyroxene, phlogopite, plagioclase and ilmenomagnetite from bottom to top in this order. Since the plagioclase is always albite, the rocks have been called "albite diabase or albite gabbros" in Finland, but we prefer here the term "karjalites' to cover the rock types of this sill in general. The sill has all the features of a layered intrusion, forming a rock series from wehrlite through clinopyroxenite and gabbro to granophyre below an upper layer of cumulates and a chilled margin against the Archaean granite. Enrichment of PGE occurs in a disturbed zone between the wehrlite and the clinopyroxenite.
General geology
The Koli layered sill (Fig 25) is located on the north eastern edge of the North Karelia Schist belt, lying concordantly with the Proterozoic quartzites but in the Archaean basement granitoids, in distance of about 200 m from the unconformity between the granitoids and quartzites. The surface section of the sill is about 60 kilometres long and it is approximately 350 m thick. It has been estimated to cover hundreds of square kilometres in area. The Archaean basement granitoids surrounding the sill are tonalites, granodiorites and granites, which vary in age from 2.9 Ga to 2.6 Ga, while the Proterozoic sequences discordantly overlying the basement date from about 2.3 Ga to 1.97 Ga, beginning with Sariolan glacigenic formations (Marmo & Ojakangas, 1984) and continued with Jatulian sequences (Vesivaara Fm, Koli Fm, Jero Fm, Puso Fm after Kohonen et al., 1992), and ending with Kalevian metaturbidites. Deposition of the Jatulian sequences was accompanied by the 2.2 Ga magmatism (Vuollo et al., 1992), and the emplacement of the the Koli sill. The Koli sill and the Jatulian sequences are intersected by 2.1 Ga and 1.97 Ga tholeiitic diabase dykes (see Pekkarinen, 1979 and Vuollo et al., 1992, respectively). Deposition of the Kalevian turbidites was followed by mantle diapirism, as evidenced by the Outokumpu ophiolite complex (Vuollo & Piirainen, 1989). Finally, all the Karelian deposits were deformed during the Svekofennian orogeny about 1.9– 1.8 Ga ago. The metamorphic degree reached the lower amphibolite facies in the eastern part of area, while the higher amphibolite facies was dominant in the western part, where granitoids of age 1.9–1.8 Ga intruded (Huhma, 1975).
37 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 25. General geological map of the eastern part of the North Karelia schist belt and the excursion site at Kaunisniemi on the Koli sill.
Internal stratigraphy
The internal stratigraphy of the sill is similar to that of many layered intrusions, including Marginal zones, Layered series and granophyre. The lower Marginal zone against the footwall contact has not been found, but the other units, including the upper Marginal zone, are clearly visible (Figs. 26 and 27). The Layered series contains different ultramafic and gabbroic rocks. The lowermost parts are composed of olivine (-chromite) and olivine-clinopyroxene cumulates. These rocks are then followed stratigraphically by clinopyroxene, clinopyroxene-magnetite and plagioclase- clinopyroxene-magnetite cumulates in ascending order. The last-mentioned grades upwards into granophyre which is found below the upper Marginal zone. The upper Marginal zone separates the sill from the Archaean granitoids, the external contact being very sharp. It is composed of a fine-grained chilled margin, noncumulate-textured pyroxenitic rocks, and clinopyroxene cumulates. The differentiation of the Layered series is normal from bottom to top but 'reversed' in the upper Marginal zone. The contacts between the layers are relatively sharp, except for those between the gabbro and granophyre and between the granophyre and the upper Marginal zone.
38 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 26. Geological map of the Koli sill from the Kaunisniemi section and the stop numbers.
Cryptic layering (Fig. 27) is seen in a restricted form in the ferromagnesian silicates of the Layered series, which are predominantly magnesium-rich in composition. One distinctive feature throughout the sill is the presence of albitic plagioclase.
Petrography and mineralogy
The Layered series begins with a 50 to 70m thick wehrlite layer, the rocks of which are composed of olivine (-chrornite) cumulates in which clinopyroxene (altered to tremolite) and brown, edenitic hornblende (Table 1; 14) occur as intercumulus minerals and form oikocrysts several centimetres in diameter. The Fo content of the olivine varies from 74 to 81 (Table 1; l-4). Chromite occurs in euhedral grains which have been altered to Fe-chromite (Table 1; 5-8). Intercumulus apatite (Table 1; 23-24) is preserved in the lowest part of the wehrlite layer. In the terminology of Irvine (1982) the olivine (- chromite) cumulates are poikilitic orthocumulates. The upper part of the wehrlite is heterogeneous and includes alternating layers of olivine (-chromite ) and olivine-clinopyroxene cumulates, which exhibit a rhythmic layering pattern in some places. The primary minerals, excluding the intercumulus edenite (Table 1; 15), have been altered to secondary minerals.
39 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 27. Detailed stratigraphy of the Koli layered sill at Kaunisniemi. Solid line = cumulus mineral, dashed line – intercumulus mineral, plus sign = subsolidus mineral. Circled numbers depict excursion stop numbers.
The wehrlites grade upwards rapidly to clinopyroxene mesocumulates. The clinopyroxenites contain fresh, well preserved diopsidic clinopyroxene as cumulus crystals (Table 1; 19–21), and the intercumulus minerals are albitic plagioclase (Table 1; 10) edenitic hornblende (Table 1; 16) and ilmenomagnetite. Ilmenomagnetite becomes the cumulus mineral in the upper part of the clinopyroxenite layer and reaches over 20 %, and apatite (Table 1; 25) appears as a cumulus mineral alongside it. The magnetite clinopyroxenite is approximately 30 m in thickness.
40 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Table 1. Mean chemical composition of minerals from Kaunisniemi section.
The contact between the magnetite clinopyroxenite and magnetite gabbro layers is marked by a sudden increase in the proportion of plagioclase (Table 1; 11) as a cumulus mineral. Other cumulus phases in the magnetite gabbro are augitic clinopyroxene (Table 1; 22) and ilmenomagnetite (Table 1;9). In the upper part of the magnetite gabbro is encountered magmatic lamination and also weak rhythmic layering. The magnetite gabbro, rich in plagioclase, grades upwards to granophyre. The rock contains large amounts of albitic plagioclase (Table 1; 12) and quartz (< - 25 %), forming a granophyric texture, and also apatite (Table 1; 26), zircon and allanite as accessory minerals.The granophyre is about 10 m thick. The total thickness of the upper Marginal zone is approximately 10 m. It begins with a fine-grained chilled margin against the Archaean granite and grades rapidly to subophitic, noncumulate-textured pyroxenite and on to coarse-grained cumulus-textured clinopyroxenite. Except for the edenitic hornblende (Table 1; 18) the primary minerals have been altered in the upper Marginal zone.
41 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Geochemistry
Selected chemical analyses for the rock types are presented in non-volatile form together with their CIPW norms in Table 2. In order to illustrate the trends in differentiation, the results of the analyses are also plotted on diagrams in Figs. 28–30. Fig. 28 shows variations in the modified differentiation index (von Gruenewaldt, 1973) and in MgO and Cr content as a function of stratigraphic height. The MDI indicates a distinct reversed fractionation trend for the upper Marginal zone, while the trend for the Layered series is normal, from bottom to top. The extent of the variation, from 20 to 80, shows unusually marked differentiation considering the thickness of the sill. The same fractionation trends are detectable in the magnesium and chromium variations as in the MDI.
Figure 28. Variations in MDI and MgO and Cr content in the rocks of the Kaunisniemi section, plotted against stratigraphic height.
The results presented in the CMA diagram (Fig. 29a) define an almost continuous, strongly curved trend from the magnesium corner to the aluminium corner. The line to the calcium corner is due to the dominant role of clinopyroxene among the cumulus phases. The AFM (Fig. 29b) and * * Al2O3 vs. (FeO /(FeO +MgO) diagrams (Fig. 29c) show that all the samples analysed are situated in the tholeiitic field, although differing from the normal tholeiitic trends. The cation plot of Jensen (1976) for the Koli sill (Fig. 29d) projects along a trend line lying above the komatiitic and normal tholeiitic suites, because of an enrichment of iron and depletion of aluminium in the karjalites. The chondrite-normalized rare earth element patterns are seen in Fig. 30. Overall rare earth element concentrations in the cumulates, the granophyre and the chilled margin of the Koli layered sill are consistent with the light rare earth element-enriched character of the parental magma.
42 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 29. Analyses of the rocks of the Kaunisniemi section plotted on a) CMA, b) AFM, c) Al203 vs. FeO*/( FeO*+MgO) and d) Jensen's diagrams.
The composition of the chilled margin (Table 2; 9) indicates the parent magma of the Koli sill. This is in equilibrium with an olivine of Fo82, which corresponds to the olivines in the wehrlite layer. The calculated mg' value for the liquid in the Koli sill is 0.59.
Platinum group elements (PGE)
A PGE concentration has been found between the wehrlite and clinopyroxenite layers (Table 3: 5), where the olivine (-chromite) and olivine-clinopyroxene cumulates form a rhythmic layering pattern. In addition to the host, the zone contains small amounts of sulphides (pyrrhotite and pentlandite) and PGE minerals (<< 10 µm), among which only froodite (Table 4; 1,2) has been identified. The concentration between the wehrlite and clinopyroxenite layer is not high, only 156 ppb in the richest sample. The samples are enriched in Os, Ir, Ru, Rh, and especially in Pd, compared with the chilled margin, while Pt is depleted (Fig. 31a and b). It is also significant that the S content rises, but Ni, Cu and Cr remain at the same levels as in the adjacent cumulates (Fig. 32). The (PGE)N pattern of the sill has features which are typical of primitive magmas (see Barnes et al. 1985).
43 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 30. REE pattern for cumulates from the Kaunisniemi section.
Conclusions
The Koli sill is an intrusion about 350 m thick and several hundreds of square kilometres in area lying concordantly with Proterozoic formations in the Archaean basement, at a distance of about 200 m from the unconformity. The intrusion has features in common with layered intrusions composed of Marginal zones, Layered series and granophyre. The parent magma of the sill, analysed from the upper chilled margin, is practically tholeiitic, but differs in being rich in iron and poor in aluminium. The volatile content and fO2 were very high compared with normal tholeiitic magma. The parent magma was strongly differentiated the MDI varying from 20 to 80. The differentiation is "reversed" in the Marginal zone but normal in the Layered series, proceeding from wehrlite (o(c)Cah, oaCh) through clinopyroxenite (aCp, amCp) to gabbros (apmC, pamC) and granophyre below the upper Marginal zone. PGE mineralization has occured between the wehrlite and clinopyroxenite in the disturbed zone, where oC and oaC form a rhythmic layered pattern. The clinopyroxene is strongly altered to amphibole in this zone and below it, but is fresh above it. The mineralized samples contain olivine, clinopyroxene and edenitic hornblene and in addition to these ilmenite, Fe-chromite, polyrnineralic sulphides (pentlandite-pyrrhotite-magnetite) and froodite.
44 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
The PGE content is low (156 ppb) in the richest sample, which is enriched in Os, Ir, Ru, Rh and Pd and depleted in Pt compared with the upper chilled margin. The (PGE)N pattern has features typical of primitive magmas. Alteration of clinopyroxene to tremolite only below the mineralized zone proves that the fluid pressure was high during crystallization of the wehrlite layer but suddenly decreased before the cumulation of the clinopyroxenite layer. This may indicate a mapa eruption and fluids boiling from the sill. This boiling will have reduced the solubility of the sulphur in the magma, causing sulphide and PGE secretion.
Figure 31. Chondrite normalized PGE pattern for a) the chilled margin and cumulates, b) the zone enriched in PGE and its adjoining cumulates.
STOP 5.1. Olivine (-chromite) cumulate ( Fig. 33a)
A fairly well preserved olivine(-chromite) cumulate is visible in the lower part of the exposure (Table 2; l), containing intercumulus clinopyroxene and edenitic hornblende. The same cumulate in the upper parts contains larger amounts of poikilitic clinopyroxene, which has been altered to an amphibole containing opaque pigment, as seen in the form of dark, raised nodules on the weathering surface. The other intercumulus minerals are phlogopite, apatite, ilmenite and small amounts of sulphides. The Fo content of the olivine is approx. 75 % in the lower part and increases to approx. 80 % by the middle of the series. The olivine has been altered, chiefly pseudomorphically, to lizardite and antigorite, but - < 20 % of it has remained unaltered in places, while the cumulus chromite has been altered to Cr-magnetite. There is scarcely any primary clinopyroxene remaining, it having virtually all been altered to amphibole and dark pigment. Large amounts of the magmatic brown amphibole have been preserved, however, and it is only in places that it has been altered to green or colourless amphibole. The olivine cumulates are texturally poikilitic orthocumulates.
45 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Table 2. Selected whole rock and trace element composition for the rock types together with their CIPW norms and MDI indexes in the Koli layered sill.
STOP 5.2. Layered olivine (-chromite) (o(c)Cah), olivine-clinopyroxene (oaCh) (Fig. 33b) and clinopyroxene (aCph) cumulates
The olivine-clinopyroxene cumulates (Table 2; 2) occur in a rhythmic unit of up to 2 m in thickness in the uppermost part of the wehrlite layer, alternating with the olivine (-chromite) cumulates. No primary minerals have been preserved in either with the exception of edenitic hornblende. The olivine-clinopyroxene cumulates are practically black in colour at their weathering surface and the olivine (-chromite) cumulates brown. This exposure also demonstrates the sharp nature of the contact or unconformity between these cumulates and the dark green, almost entirely unaltered clinopyroxene cumulate layer. Slightly elevated PGE concentrations have been found about 1 m below the oC / aC contact in the lower part of this rhythmically layered unit (Fig. 31; samples 30 and 463), and froodite has been identified in one sample from the same unit (Table 4).
STOP 5.3. Clinopyroxenite (aCph) (Fig. 33c)
A pure clinopyroxenite (aC, Table 2; 3) in which the primary clinopyroxene has been preserved practically unaltered is to be found in the lowermost parts of the series, where it has the
46 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
composition of a diopside-endiopside. Texturally this rock is a mesocumulate, the lower parts of which contain up to 90 % clinopyroxene. The intercumulus materia1 consists of an albitic plagioclase, which shows up as a lighter coloration at the weathered surface, and a little ilmenomagnetite and edenitic hornblende. The plagioclase in the lowermost parts is well preserved and has only a little epidote on its surface.
Tables 3 and 4. PGE and Au analyses (ppb).
Figure 32. Variations in S, Cu, Ni, Cr and PGE content in the wehrlite (o(c)C/oaC) and lower parts of the clinopyroxenite layer.
47 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
STOP 5.4. Magnetite clinopyroxenite (amCph) and magnetite gabbro (apmC) (Fig. 33d)
Located above the clinopyroxenites (aC) are magnetite clinopyroxenites (amC, < 50 m) in which ilmenomagnetite can account for up to 20 %. These magnetite clinopyroxenites (Table 2; 4) contain very much larger amounts of chalcopyrite and pyrite than the other units. Primary clinopyroxene has been encountered in the extreme upper parts of the magnetite clinopyroxenite, but has been entirely altered to secondary amphibole further up in the series. The plagioclase is also fairly extensively altered and contains large amounts of epidote. The other intercumulus minerals are edenitic hornblende and sulphide ore minerals. The magnetite clinopyroxenites (amC) are sharply divided from the magnetite gabbros above them (apmC, Table 2; 5), in which plagioclase appears in the cumulus facies and increases markedly in amount, to >50 %. This rock has once contained approx. 40 % clinopyroxene, but it has now been entirely altered. Magnetite drops in amount to 5–10 % at this point. The intercumulus minerals are edenitic hornblende and sulphides, and a little apatite is found as a cumulus mineral.
STOP 5.5. Layered and laminated magnetite gabbro (pamC) (Fig. 33e)
This rock is mineralogically similar to the magnetite gabbro above but here presents distinct signs of magmatic lamination. The plagioclase and pyroxene grains in the cumulate are oriented in the direction of the sill, and a rhythmic layering is detectable, with alternating layers richer in plagioclase and pyroxene.
STOP 5.6. Granophyre (Fig. 33f), the upper Marginal zone (upper clinopyroxenite, Fig. 33f) and chilled margin (Fig. 33g) and basement granite
The coarse-grained magnetite gabbro is seen to be overlain by approx. 10 m of a quartz-rich granophyre (Table 2; 7) with a maximum content of 25 % quartz. The albitic plagioclase is also coarse-grained, and accounts for 60–80 %. Typical of this unit is the presence of quartz-feldspar 'granophyric' intergrowths. Other minerals present are biotite, allanite, apatite, ilmenite, some sulphides, and edenitic hornblende. The granophyre grades steadily to a pegmatoid clinopyroxenite (Table 2; 8) with maximum grains of 10 cm, which forms the lowermost part of the upper Marginal zone. The clinopyroxene has been entirely altered to amphiboles (actinolite and hornblende), and the intercumulus plagioclase to coarse-grained epidote. Small amounts of apatite, edenitic hornblende, ilmenite and pyrite are also found. A fine-grained chilled margin (Table 2; 9) and subophitic pyroxenite of a non-cumulate texture occur at the contact with the Archaean granitoids. The principal minerals are a green actinolite hornblende and an albitic plagioclase.
48 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Fig. 33. a) Olivine-(chromite) cumulate, 26-JIV-85, 24.30 m, b) the contact between olivine-(chromite) and olivine-clinopyroxene cumulates, 33B-JIV-85, 36.20 m, c) clinopyroxene curnulate, 35-JIV-85, 48.00 m, d ) the sharp contact between clinopyroxene-magnetite and clinopyroxeneplagioclase- magnetite cumulates, 43-JIV-85, 231.60 m, e) laminated plagioclase-clinopyroxene-magnetite cumulate, 47-JIV-85, 263.40 m, f) granophyre, 51-JIV-85, 312.60 m, g) upper clinopyroxenite, 53-JIV- 80, 325.40 m, h) chilled margin, 430-JIV-80, 330.00 m.
49 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
STOP 5.7. 1.97 Ga dyke
The ~1.98 Ga dyke swarm is not as voluminous as the ~2.1 Ga dyke swarm (Vuollo, 1994). However, recent geochronological (Vuollo & Huhma, 2004) and field studies, combined with aeromagnetic data, show that the ~1.98 Ga dykes are found throughout the Archean basement and the Karelian formations. The swarm consists of up to 70 m wide dykes that form prominent NW- trending (320–340°), linear features >120 km in length (Kuhmo block). Tholeiitic dykes of ~1.97 Ga age have been found in connection with Proterozoic quartzites in the Koli area (Vuollo et al., 1992). The dykes are only slightly altered and contain primary olivine, pyroxenes, and plagioclase and have an ophitic texture. The zircon age (1965±10 Ma) is supported by the Sm-Nd mineral age of 1985±80Ma. The 1.97 Ga dykes include both quartz and olivine normative compositions. They are lower in Fe, P, Ti, Zr, and Y and higher in Mg, Cr, and Ni than the 2.1 Ga dykes in the Koli region. Mg´ values range from 71 to 49. The REE-patterns show slight enrichment with (La/Yb)N= 1.6–2.6 and REE concentrations range from 5 to 28 times chondrites. Petrogenetic modelling implies that the dykes have been generated by 20–30% partial melting of a LREE-enriched mantle peridotite. The diabases have slightly positive initial ,Nd values (+1.8) and show geochemical affinitities to island arc tholeiites.
STOP 5.8. The Finnish Stone Centre, Juuka
The Finnish Stone Centre aims at increasing the recognition of Finnish natural stone, promoting the competitiveness of the natural stone sector, and launching new businesses. The world of stone is introduced from the viewpoint of art, science and construction. Exhibition–“Of Many Forms” is an exhibition that tells how Finnish design has revised and renewed sculpture. Though internationally renowned, Tapio Wirkkala and Timo Sarpaneva, as designers, were not accepted in the field of the arts. Nonetheless, they had strong impact on the development of modern sculpture in Finland. The new concepts emerged in the works of Kain Tapper, among other sculptors. The works on display are from the prime creative period of the artists. The exhibition focuses on their sculptural works in wood, glass, bronze and stone. The works combine the features of Finnish nature and peasant craft traditions with international modernism. Soapstone art–In the span of several decades, German honorary consul Karl Heinz Arnold has gathered a unique and extensive collection of soapstone art. The collection includes over 6000 miniature sculptures, of which more than 460 items are displayed at the Finnish Stone Centre. The items in the collection originate from all around the world: human figurines from the Congo, Inuit sculptures portraying hunting scenes, oil lamps from Afghanistan and home altar vases from China. The items on display include cylindrical seals and printed clay tablets from 2950 BC that were found in the area currently known as Iraq. The oldest of the works date from over 5000 years ago. Stone Shop–The Stone Shop at the Finnish Stone Centre offers a wide variety of stones and stone products for sale. Their assortment includes rough stones, soapstone products by Hukka Design, stone jewellery, gifts and utility articles as well as ceramics. Curiosities include petrified wood, sand roses, fossils and dinosaur bones. Stone Park–The Stone Park is an oasis in the Stone Village. The various terraces of the park have been bordered with pavings and groups of plants while taking pre-existing vegetation into consideration. The Stone Centre faces the park, and a shallow pool has been constructed in front of the main façade. Between the pool and the river lies an artificial island, which has been landscaped using earth and rubble. The Stone Park has been designed in co-operation with teams of students from the University of Art and Design, Helsinki (Taideteollinen korkeakoulu), and from North Karelia Polytechnic (Pohjois-Karjalan ammattikorkeakoulu). Studio A&G Mining–Studio A&G creates Finnish jewellery in the Finnish Stone Centre in Nunnanlahti, Juuka. The jewellery selection has been inspired by the nature of Eastern Finland,
50 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
where the jewels used for them also originate from: diamonds, garnets, diopsides and ilmenites from the municipality of Kaavi. The jewellery has been created by the prominent Finnish designers Marja Kurki, Kaisa Blomstedt, Jussi Louesalmi and Timo Nupponen. Special mention is due to Outi Toivanen, a talented young designer working for the company whom one can see at work every day in our studio at the Stone Centre. Individual pieces of jewellery for the company have also been designed by Finnish celebrities Tommy Tabermann, Kata Kärkkäinen, Maarit and Sami Hurmerinta, Åke Lindman and Juice Leskinen.
2.1 Ga Fe-tholeiitic dykes of Varpaisjärvi
Jorma Paavola Geological Survey of Finland, P.O.Box 1237, FI-70211 Kuopio
Early Proterozoic, NW-trending mafic dykes are abundant in the Iisalmi block. In areas where they cut the Archean basement granulites, the primary magmatic mineralogy is often well- preserved, presumably due to the dry mineralogy of the wall-rock granulites. Early Proterozoic diabase dykes are abundant in the Sonkajärvi–Varpaisjärvi area, Central Finland (Fig. 34). The dykes have intruded into the Archaean (3.1–2.7 Ga) bedrock. The predominant strike of the more or less vertical dykes is NW-SE and their width varies from some meters up to ~150 meters. Based on 50 elemental geochemical dyke analyses across the area, the 2.1 Ga dykes comprise a relatively evolved, coherent group which is characterised by tholeiitic trends: Concentrations of TiO2 (0.8–2.5 %) and Fe2O3tot (11–19 %) increase and those of Al2O3 (16–11 %) and CaO decrease (12–8 %) along with decreasing MgO (8–4 %). These correlations, the positive correlations between MgO, Ni, and Cr, and the nearly constant SiO2 contents (48–51 %) are readily ascribed to fractional crystallization of a gabbroic (plagioclase, clinopyroxene, olivine, Cr-spinel) mineral assemblage (Toivola, 1988). Chondrite-normalised REE-patterns indicate variably LREE- enriched compositions; (La/Yb)N ranges typically from 1.1 to 2.5 but LREE-depleted patterns with (La/Yb)N of 0.48 have also been found. The dykes show affinities to oceanic basalts in terms of TiO2-K2O-P2O5-Zr -ratios and -concentrations. Their overall compositional uniformity suggests derivation from a common parental magma.
STOP 5.9. Varpaisjärvi dyke
The Nieminen dyke in the Jonsa area, Varpaisjärvi, is a good example of the 2.1 Ga dykes. Dykes within the Jonsa granulite block are abundant and strike ~W; elsewhere in the Iisalmi– Varpaisjärvi area the predominant strike is roughly NW. The Nieminen dyke is well-exposed in a presently abandoned dimension stone quarry. Both contacts are visible; the width of the dyke is 25–30 m. The dyke is homogeneous and unaltered. The main silicate constituents are plagioclase and clinopyroxene and minor orthopyroxene. Secondary minerals, such as light-green amphibole, biotite, and epidote occur sporadically on the grain surfaces. Geochemically, the Nieminenn dyke is a subalkaline, tholeiitic basalt and similar to the other dykes in the region with e.g. TiO2 of 1.2 % and MgO of 6.2 %. Repeated (n=12) Sm-Nd isotopic analyses of minerals in sample A1223 from the quarry yield an isochron that corresponds to an age of 2127±42 Ma and an initial ,Nd of +2.5. In places, the dyke contains relatively more coarse- grained and more plagioclase-rich patches (sample A1368). A small amount of zircon was obtained from such material and facilitated eight U-Pb analyses. The results have been controversial: A few brownish, long crystals with relatively low density yielded a slightly discordant 207Pb/206Pb age of 1866 Ma. The relatively heavier zircons yielded variably discordant but consistent 207Pb/206Pb ages of ~2.1 Ga; rejection of one poor analysis results in a chord with intercepts at 2100±3 Ma and - 256±150 Ma (MSWD=0.85). The average 207Pb/206Pb age of 2106±6 Ma provides the best estimate
51 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
of the age of magmatic zircon. The few brownish zircon grains with significantly younger 207Pb/206Pb-age are quite exotic and form only very minor population (all grains were used for analysis C). It remains obscure whether there have been some later veining. The geochronological studies of the Nieminen dyke indicate that the intrusion of the Nieminen dyke, and, presumably, the other Fe-tholeiitic dykes in the region, took place at ~2.1 Ga. The positive intial ,Nd value (+2.5) indicates that the magma was derived from a depleted mantle source (,Nd (2.1 Ga) of DM is +3.3) and precludes strong interaction of the magma with the Archaean granulite wall-rock, which has ,Nd (2.1 Ga) values on the order of –10 (leucosome in mafic granulite) to –6 (Varpasijärvi enderbite) (Hölttä et al., 2000).
Figure 34. Sketch map showing the distribution of the diabases in the Sonkajärvi– Varpaisjärvi area. The locality of the Nieminen dyke is depicted by the square. Symbols: 1. Archaean amphibolite-banded tonalitic–trondhjemitic migmatite or granitoid / corresponding hypersthene-bearing rock. 2. Proterozoic intrusive/medasediment. 3. Fracture or fault. 4. Diabase dyke. (Toivola et al., 1991)
52 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Day 6: Jormua ophiolite and the oldest rock in Europe
Dykes in the Jormua ophiolite
Asko Kontinen Geological Survey of Finland, P.O.Box 1237, FIN-70211 Kuopio
The Jormua Ophiolite is an allochthonous mafic-ultramafic rock complex that at ~1.9 Ga was thrusted onto the Archaean craton margin, from a formative setting within a passive margin environment ~100 km southwest from its present position (Kontinen, 1987) (Fig. 35). The complex exposes a unique fragment of Proterozoic Red Sea-type oceanic crust. The complex comprises two distinct units: 1) fragments of Archaean subcontinental lithospheric mantle that became exposed from beneath the Karelian craton by detachment faulting, following its complete break-up (Peltonen et al., 2003), and 2) alkaline and tholeiitic igneous suites that were emplaced within and through the lithospheric mantle at ~2.1 Ga and 1.95 Ga, respectively. At the prerift stage of the continental breakup (~2.1 Ga), residual lithospheric peridotites became intruded by alkaline melts that formed “dry” clinopyroxene cumulate dykes. Slightly later, this same piece of mantle became extensively intruded by hydrous alkaline magmas that resulted in formation of high-pressure hornblendite-garnetite cumulates deep in the ophiolite stratigraphy and fine grained OIB-type dykes at more shallow levels. Simultaneously, the residual peridotites became metasomatized due to porous flow of the melt in the peridotite matrix. The alkaline magmatism was soon followed by lithospheric detachment faulting that exposed the subcrustal peridotites at the seafloor, where they at 1.95 Ga became covered by tholeiitic (E-MORB) pillow and massive lavas and intruded by coeval, tholeiitic (E-MORB) dykes and gabbros. Since transitional contacts between all main ophiolite units can be demonstrated, the Jormua Ophiolite Complex is interpreted to represent a practically complete sample of seafloor from an ancient ocean-continent transition (OCT) zone, strikingly similar to that reported from younger similar tectonic settings, such as the Cretaceous West Iberia nonvolcanic continental margin.
STOP 6.1. Serpentinite and clinopyroxenite and hornblendite-garnetite dykes
The western block of Jormua ophiolite complex, best exposed at Hannusranta, comprises a 0.5–1.5 km wide, over 10 km long sliver of lherzolitic-harzburgitic mantle peridotites transformed into antigorite±olivine metaserpentinites (Fig. 36). The peridotites and mantle foliations in them are truncated by abundant clinopyroxenite and hornblendite dykes. Some of the hornblendite dykes comprise (locally abundant) garnet pseudomorphs. Two distinct subtyps of clinopyroxene- amphibolegarnet mantle dykes can be distinguished on the basis of their cross-cutting relationships, colour of weathering surfaces, grain size and mineral modes. Individual Type A mantle dykes (clinopyroxenite) are generally 0.2–1 m wide, locally forming dyke-in-dyke sets up to >10 m wide. They were originally medium-grained “dry” clinopyroxene ortho- or mesocumulates that contain subordinate amounts of Al-rich calcic amphibole. Most of the Type B mantle dykes are coarser grained (typically >10 mm) than Type A dykes, being “hornblendites” originally with or without garnet. Less common Type B subtypes include pegmatitic varieties with amphibole up to 10 cm, garnetite dykes, and magnetite+ilmenite+zirconolite-rich clinopyroxene cumulates. Single zircon SIMMS and Sm-Nd isotope data from the clinopyroxenite dykes suggest they emplaced ~1.95 Ga ago and that the host mantle would be a piece of late Archaean (>3 Ga) subcontinental lithospheric mantle (Peltonen et al., 2003). All the main types of mantle dykes in the western block of the Jormua ophiolite will be seen. The Hannusranta stop requires 1.5 km of walking, partly along a clear path, partly in a bushy, rocky terrain. Waterproof boots are recommended.
53 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 35. Generalised geological maps of the central part of the Fennoscandian Shield (inset) and the Jormua Ophiolite. Suture zone on the inset map indicates the approximate location of the tectonic boundary between the 2.7–3.1 Ga craton and the 1.88 Ga Svecofennian arc complex.
STOP 6.2. Serpentinite and “OIB” metalamprophyre and “E-MORB” dykes
The Lehmivaara outcrops are located in the middle part of the central block of the Jormua ophiolite (Fig. 36). Characteristic for the middle block are myriads of metadolerite to metabasaltic dykes of E-MORB chemical affinity. The E-MORB dykes frequently occur in metres to tens of metres wide dyke-in-dyke sets, inside of which the individual dykes are mostly 20–120 cm wide. Geochemically, they resemble the pillow and massive lavas of the Jormua Ophiolite (Fig. 37a,c), but have notably lower SiO2 and alkali contents, and high MgO and FeO* values (Peltonen et al., 1996). These differences, however, have been ascribed to interaction with the wall-rock serpentinites. Based on Zr/TiO2 vs. Nb/Y relationships, all of the E-MORB dykes are subalkaline basalts. In addition, there are older, markedly LREE-enriched dykes (Fig. 37d), being crosscut by the E-MORB dykes. These “OIB” dykes have high Nb/Y values and are usually discrete (5–150 cm wide) and far fewer in number than the E-MORB-dykes. They are strongly altered but probably mostly of ultramafic lamprophyre origin. SIMMS zircon data from the OIB dykes demonstrate presence of Archaean zircons, probably inherited from the host peridotites. An origin as Archaean SCLM seems likely also for the Jormua central block peridotites. Doing the stop 2 involves 2 km walking, partly along a forest road, partly in an open-cut forest. Sturdy shoes or boots are recommended. Given a sunny day, an added bonus at this stop is a wide, blue view over the eastern part of the lake Oulunjärvi, the forth-largest lake in Finland.
54 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Figure 36. Stratigraphic reconstruction of the Jormua Ophiolite. The western block, extensively intruded by clinopyroxene+amphibole±garnet mantle dykes, is compositionally and structurally distinct from the other main blocks.
Figure 37. Chondrite-normalised REE-patterns for the E-MORB-type (“Main Suite”) pillow lavas (a), sheeted dykes (b), and “deep dykes (c) and the OIB dykes (“Early dykes”) (d). Inset in (b) illustrates the fractionated character of an anomalous dyke sample.
55 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
STOP 6.3. Sheeted dykes
After mantle peridotites, sheeted dykes are the second most voluminous lithological component in the Jormua Ophiolite. Large outcrops composed 100% of mostly 20–120 cm wide metadolerite-metabasalt dykes are present in the Sammakkolampi area. The dykes are of E-MORB tholeiite composition (Fig. 37b), partly plagioclase phyric, partly nearly aphyric. Most of the classical features of sheeted dyke units of ophiolites are seen in the Sammakkomäki outcrops. There are several generations of broadly subparallel dykes, the older being intruded and split by the younger resulting in “marginless” (septa without chill margins) and “half” (septa with one chill margin) dykes. Branching of the dykes and apophyses along their contacts are common. All these features are well visible due to the nicely ice polished nature of outcrops at Sammakkomäki. The Sammakkolampi stop poses a walk of 2 km, for most part along a forest road. Sturdy shoes make the visit comfortable.
STOP 6.4. Harzburgite and metarodingitic gabbro dyke
Jormua peridotites are all pervasively altered and metamorphosed to mostly nonpseudomoprphic antigorite serpentinite. In these metaserpentinites, the only mantle mineral preserved, to some extent, is chromite. The Kontiomäki Shell outcrop is an example of those rare Jormua metaserpentinites that still show some pseudomorphic mantle peridotite textures. Pale-grey coloured spots in the weathering surface of the outcrop are pseudomporps after orthopyroxene, while the intervening, slightly darker coloured serpentine mass is after olivine. In thin sections of the metaserpentinite it can be seen that the orthopyroxene pseudomorps comprise bastite replaced by antigorite, and that the intervening antigorite mass contains remains of magnetite dust meshes typical of low-T serpentine after olivine. Oxide grains, largest of which have chromite cores, define a weakly developed foliation of an obvious mantle origin. A metre or so thick very coarse-grained gabbro dyke crosscuts the harzburgite. Typical of lower amphibolite facies metarodingites, the gabbro dyke consists of clinopyroxene (diopside) variably replaced by amphibole, while the original magmatic plagioclase has been replaced largely by epidote minerals. In nearby outcrops similar metarodingic gabbro dykes comprise locally abundant grossular garnet. A U-Pb zircon age of 1953±2 Ma has been obtained for the mantle gabbro dykes of the Jormua eastern block. The stop 6.4. outcrop is located just 100 m to the south of Shell Kontiomäki, our luncheonette while visiting Jormua.
STOP 6.5. Gabbros, ferrogabros and plagiogranites
A thick unit of gabbros, layered mafic-ultramafic for its basal parts, which is characteristic of many classical ophiolites, is distinctly missing in the Jormua Complex. Instead all gabbro occurrences in Jormua represent stocks and dykes intrusive into the mantle unit. Best outcrops to study the variety of the Jormua gabbros and related intermediate-felsic rocks are at Sarvikangas locality within the western block of the ophiolite. Eastern part of the Sarvikangas area comprises greyish green, coarse-medium grained gabbros intruded by many metadolerite-basalt dykes, often in dyke-in-dyke sets and frequently showing chill margins against the host gabbros. In the western part of the area gabbro, ferrogabbro and minor microgabbro-diorite-leucotonalite (plagiogranite) are found in close association. The gabbros are mostly of “isotropic” character, while the ferrogabbros range “isotropic” to “varied- textured” (heterogenous, coarse-pegmatoid). Transitions between the gabbros, ferrogabbros and microgabbro-diorite-leucotonalites are mostly abrupt. There are outcrop features that suggest emplacement of the microgabbro-diorite-leucotonalite suite was a multistage progressive process involving mingling and various pulses of magma.
56 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
Mineral assemblages in Jormua gabbroic rocks are usually wholly metamorphic, with the exceptions that occasionally relicts of primary An-rich plagioclase are preserved in gabbros and primary ilmenite commonly in ferrogabbros. The diorite-leucotonalites at Sarvikangas have the chemical characters of ocean ridge granites being e.g. very high in Na2O but very low in K2O and high in Y and Nb. Zircons from one leucotonalite sample from Sarvikangas yield a somewhat unprecise U-Pb age of 1954±12 Ma. Nd (1.95 Ga) for this sample is +1.9, noting that Jormua lavas and dykes yield an average Nd (1.95 Ga) of +2, this suggests that the Sarvikangas gabbroic and plagiogranitic rocks were intimately related to the 1.95 Ga oceanic crust-forming process. Doing the stop 2 involves about 1 km walking in a bushy, rocky terrain. Once again, sturdy shoes or boots are recommended.
STOP 6.6. Podiform chromitite
Typical of nonsuprasubduction zone ophiolites, chromitites are rare in Jormua ophiolite. They have been located only within one small peridotite slice (200x700 m) at Pitkänperä, which comprises a few small pods of massive chromitite. Stop 5 has to show the largest of the known pods, which is ca. 1 m thick and at least 5 m long. The core part of the Pitkänperä pod consists of coarse-grained well-preserved, moderately aluminous chromite (25% Al2O3, 45% Cr2O3), while chromite at the margins of the pod show heavy fracturing and alteration into carbonate and chlorite. Some of the larger chromites in the core of the pod preserve tiny, roundish inclusions of sodic tshermakitic amphibole, probably after melt inclusions. The host peridotite shows intense pre- peakmetamorphic carbonate alteration. This is a roadside stop.
STOP 6.7. Pillow lavas
The extrusive unit of the Jormua ophiolite comprises mainly pillow lavas of E-MORB composition (Fig. 37a). These are met about only in the eastern part of the ophiolite, where they occur in tectonic slices up to a couple of hundreds of metres thick and several kilometres long. The Kylmä lava slice has a maximum thickness of ca. 400m. It is underlain by upper Kaleva metawackes-black schists and overlain by a thin sheet of serpentinite-talc-carbonate rocks. The Kylmä sequence is 100% metavolcanic without any terrigenous or cherty intercalations. Most of the flows are pillowed with occasional massive basal parts. The Kylmä pillows often preserve distinct gas discharge channels and thin chill margins. Minor pillow breccia and hyaloclastite developments occur at the top parts of flows. In one locality there are clinopyroxenite-gabbro and dolerite dykes inside of the pillow lava sequence. Visting the Kylmä stop involves some 600 m of waking in an open-cut, rocky forest. During the walk some wet places may be crossed.
The oldes rock of Europe at Siurua
Katja Lalli1 & Hannu Huhma2 1Geological Survey of Finland, P.O.Box 77, FIN-96101 Rovaniemi 2Geological Survey of Finland, P.O.Box 96, FIN-02151 Espoo
The poorly known Pudasjärvi Complex consist mainly of Archaean gneisses and granitoids and abundant amphibolites. Proterozoic granites and diabase dykes have intruded the Arhcaean gneisses. The N-S trending Pudasjärvi Granulite Belt is located in the middle of the gneiss complex. It is >50 km long and is cut by several fault and shear zones into smaller pieces. The metamorphic grade between the blocks varies from amphibolite to granulite facies. West from Pudasjärvi Granulite Belt lies the Oijärvi greenstone belt with associated gold deposits.
57 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7. – 3.8.2005 – ROVANIEMI, FINLAND
STOP 1.2 The oldest rock of Europe at Siurua (3.5 Ga)
The Siurua trondhjemite gneiss is the dominant rock in a large outcrop area (Fig. 38). The rock is granoblastic, medium-grained and consists of plagioclase, quartz, potassium feldspar, dark biotite and accessory fluorapatite, zircon, monazite, ilmenite, magnetite and metamict allanite. Dark green hornblende and garnet occur in one sample. The chemical composition is close to that of average Archean TTG gneisses. The major differences are the depletion of phosphorus and strong enrichment of Th and LREE. The REE-patterns of the Siurua gneiss and felsic granulite of the Pudasjärvi Granulite Belt are similar and exhibit a negative Eu anomaly; however, the REE concentrations of the Siurua trondhjemite are about an order of magnitude higher. Gneissic banding is seen on outcrop, but penetrative deformation is not noticeable under microscope. Gneissosity- parallel veins of granite pegmatite and roundish boudins of mafic granulite appear on outcrop, although the gneiss is not in its present state on granulite facies. The zircon population has primarily a magmatic and relatively homogenous appearance. Ion- microprobe U-Th-Pb analytical data appear to be reverse discordant. Nevertheless, out of 18 analysed grains 15 yielded 207Pb/206Pb ages of 3.4–3.5 Ga (Mutanen & Huhma, 2003). The best estimate for igneous crystallization of the Siurua gneiss is ca. 3.5 Ga, which reveals that the trondhjemite gneiss is the oldest rock so far identified in the Fennoscandian Shield. Signs of an even older crustal contribution are obtained from a 3.73 Ga old zircon core.
Figure 38. Location map of Siurua showing the distribution of outcrops and the age dating site. Base map © National land survey of Finland, licence No 13/MYY/05.
58 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
References
Äikäs, O., 2000. Juankoski. Geological map of Finland 1:100 000: pre-Quaternary rocks, sheet 3333. Alapieti, T.T., Filén, B.A., Lahtinen, J.J., Lavrov, M.M., Smolkin, V.F., Voitsekhovsky, S.N., 1990. Early Proterozoic layered intrusions in the northeastern part of the Fennoscandian Shield. In: E.F. Stumpfl, H. Papunen (Eds.), Proceedings of the Fifth International Platinum Symposium Symposium, Helsinki 1989. Mineral. Petrol. 42, 1–22. Alviola, R., 1981. Selostus Mäntyharjun Lovasjär-ven intruusiota koskevista teollisuusmine- raalitutkimuksista vv. 1976 – 1978. Käsikirjoitus M 19/3132/-81/80. Unpublished report, Geol. Surv. Finland. 1–91. Alviola, R., Johanson, B.S., Rämö, O.T., Vaasjoki, M., 1999. The Proterozoic Ahvenisto rapakivi granite – massif-type anorthosite complex, southeastern Finland; petrography and U–Pb chronology. In: I. Haapala, O.T. Rämö (Eds.), Rapakivi granites and related rocks. Precambrian Res. 95, 89–107. Amelin, Yu.V., Semenov, V.S., 1996. Nd and Sr isotopic geochemistry of mafic layered intrusions in the eastern Baltic shield: implications for the evolution of Paleoproterozoic continental mafic magmas. Contrib. Mineral. Petrol. 124, 255–272. Anttila, P., Kuivamäki, A., Lindberg, A., Kurimo, M., Paananen, M., Luukkonen, E.J., Front, K., Pitkänen, P., 1991. The geology of the Veitsivaara area, summary report. Helsinki, Voimayhtiöiden Ydinjätetoimikunta, Raportti YJT–91–08, 1–42. Aro, K., 1987. Vaasan saariston ja Petolahden diabaasit. Diabases in the Vaasa archipelago and Petolahti, western Finland. In: K. Aro, I. Laitakari (Eds.), Suomen diabaasit ja muut mafiset juonikivilajit. Diabases and other mafic dyke rocks in Finland. Geol. Surv. Finland, Rep. Invest. 76, 179–184. (in Finnish with English abstract and figure and table captions) Aro, K., Laitakari, I. (Eds.), 1987. Suomen diabaasit ja muut mafiset juonikivilajit – Diabases and other mafic dyke rocks in Finland. (Appended map; Mafic dyke rocks of Finland 1:2 000 000) Geol. Surv. Finland, Rep. Invest. 76. 1–254 (in Finnish with English abstracts and figure and table captions) Barnes, S-J., Naldrett, A.J., Gorton, M.P., 1985. The origin of the fractionation of Platinum-group Elements in terrestrial magmas. Chem. Geol. 53: 303-323. Bergman, L., 1981. Berggrunden inom Signilskär, Mariehamn och Geta kartblad. Signilskärin, Maarianhaminan ja Getan kartta-alueiden kallioperä. Summary: Pre-Quaternary rocks of the Signilskär, Mariehamn, and Geta map sheet areas. Geological map of Finland 1: 100 000, Explanation to the maps of Pre-Quaternary rocks. Geol. Surv. Finland, Espoo. 1–72. Bergman, L., 1986. Structure and mechanism of intrusion of postorogenic granites in the archipelago of southwestern Finland. Acta Acad. Aboensis. Ser. B Mathematica et Physica 46 (5), 1 –74. Boyd, W.W., 1972. Diabase variation and genesis. Bull. Geol. Soc. Finland 44, 21–34. DePaolo, D.J., 1981. Neodymium isotopes in the Colorado Front range and crust-mantle evolution in the Proterozoic. Nature 291, 193–196. Ehlers, C., Ehlers, M., 1977. Shearing and multiple intrusion in the diabases of Åland archipelago, SW Finland. Geol. Surv. Finland, Bull. 289, 1–31. Ehlers, C., Bergman, L., 1984. Structure and mechanism of two postorogenic granitic massifs, southwestern Finland. In: Krfner, A., Greiling, R. (Eds.), Precambrian Tectonics Illustrated. Schwitzerbart, Stuttgart, pp. 173– 190. Eklund, O., 1993. Coeval contrasting magmatism and magma mixing in Proterozoic post- and anorogenic granites, Åland, SW Finland. Åbo Akademis tryckeri, Åbo, 1–57.
59 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Eklund, O., Shebanov, A.D., 1999. The origin of rapakivi texture by sub-isothermal decompression. In: I. Haapala, O.T. Rämö (Eds.), Rapakivi granites and related rocks. Precambrian Res. 95, 129–146. Eklund, O., Shebanov, A., 2005. Prolonged postcollisional shoshonitic magmatism in the southern Svecofennian domain - a case study of the Ava granite-lamprophyre ring complex Lithos 80, 229–247. Eklund, O., Fröjdö, S., Lindberg, B., 1994. Magma mixing, the petrogenetic link between anorthositic suites and rapakivi granites, Åland, SW Finland. Mineral. Petrol. 50, 3–19. Eklund, O., Konopelko, D., Rutanen, H., Fröjdö, S., Shebanov, A.D., 1998. 1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian Shield. Lithos 45, 87–108. Elo, S., Rastas, J., Rämö, O.T., 1996. Gravity and aeromagnetic anomaly patterns of gabbro- anorthosites associated with rapakivi granites of Southern Finland. In: I. Haapala, O.T. Rämö, P. Kosunen (Eds.), The Seventh International Symposium on Rapakivi Granites and Related Rocks, University of Helsinki, July 24–26, 1996, Abstract Volume. Helsinki University Press. 22–23. Gruenewaldt, G. Von, 1973. The modified differentiation index and the modified crystallization index as parameters of differentiation in layered intrusions. Trans. Geol. Soc. S. Afr. 76: 53-61. Haapala, I., 1977. Petrography and geochemistry of the Eurajoki stock, a rapakivi-granite complex with greisen-type mineralization in southwestern Finland. Geol. Surv. Finland, Bull. 286, 1–128. Hämäläinen, A., 1987. Satakunnan postjotuniset diabaasit. The Postjotnian diabases of Sata-kunta. Geol. Surv. Finland, Rep. Invest. 76, 173–178. (in Finnish with English abstract and figure and table captions) Hanski, E., 1982. Albiittidiabaasit ja niihin liittyvät ultramafiset kivet Kuhmon ja Kolin alueilla. Raportti No. 6, Oulun yliopisto, Arkeeisten alueiden malmiprojekti, 76 p. Hanski, E., 1984. Geology of the Gabbro-Wehrlite association in the Eastern part of the Baltic Shield. Raportti No. 20, Oulun yliopisto, Arkeeisten alueiden malmiprojekti, 78 p. Hanski, E., 1986. Gabbro-Wehrlite association in the eastern part of the Baltic shield. In: Friedrich, G.H., et al. (eds.) Geology and Metallogeny of Copper Deposits. Springer-Verlag. Berlin, Heidelberg, pp. 151-170. Hanski, E., 1997. The Nuttio serpentinite belt, Central Lapland: An example of Paleoproterozoic ophiolitic mantle rocks in Finland. Ofioliti 22, 35–46. Hanski, E., Walker, R.J., Huhma, H., Suominen, I., 2001. The Os and Nd isotopic systematics of the c. 2.44 Ga Akanvaara and Koitelainen mafic layered intrusions in northern Finland. Precambrian Res. 109, 73–102. Hölttä, P., Huhma, H., Mänttäri, I., Paavola, J., 2000. P–T–t development of Archaean granulites in Varpaisjärvi, Central Finland II. Dating of high-grade metamorphism with the U–Pb and Sm–Nd methods. Lithos 50, 121–136. Honkamo, M., 1972. Kolin - Juuan alueen ultramafiset kivilajit. M. Sci. Thesis. Dept. Geol. Univ. Oulu, 91 p. Huhma, A., 1981. Youngest Precambrian dyke rocks in North Karelia, East Finland. Bull. Geol. Soc. Finland 53, 67–82. Huhma, H., 1986. Sm-Nd, U-Pb and Pb-Pb isotopic evidence for the origin of the Early Proterozoic Svecokarelian crust in Finland. Geol. Surv. Finland, Bull. 337, 1–48. Huhma, H., Cliff, R.A., Perttunen, V., Sakko, M., 1990. Sm–Nd and Pb isotopic study of mafic rocks associated with early Proterozoic continental rifting: the Peräpohja schist belt in northern Finland. Contrib. Mineral. Petrol. 104, 369–379. Irvine, T.N., 1982. Terminology for Layered Intrusions. J. Petrol. 23, 127–162.
60 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Jensen, L.S., 1976. A new cation plot for classifying subalkalic volcanic rocks. Ontario Division of Mines, Miscellancous Paper 66. Johanson, B.S., 1984. Ahvenisto gabro-anortosit-komplex - en petrografisk och mineralogisk undersökning. M.Sc. Thesis, University of Helsinki, Finland. 1–85. (in Swedish) Kaitaro, S., 1953. Geologic structure of the late pre-Cambrian intrusives in the 2va area, 2land Islands. Bull. Comm. Géol. Finlande 62, 1 – 69. Kontinen, A., 1987. An early Proterozoic ophiolite – the Jormua mafic-ultramafic complex, northern Finland. Precambrian Res. 35, 313-341. Kontinen, A., Peltonen, P., 1998. Excursion to the Jormua ophiolite complex. In: Hanski, E., and Vuollo, J. (eds.) International ophiolite symposium and field excursion: generation and emplacement of ophiolites through time, August 10-15, 1998, Oulu, Finland. Abstracts, excursion guide. Geol. Surv. Finland, Spec. Pap. 26, 69-89. Kontinen, A., Paavola, J., Lukkarinen, H., 1992. K-Ar ages of hornblende and biotite from Late Archaean rocks of eastern Finland—interpretation and discussion of tectonic implications. Geol. Surv. Finland, Bull. 365, 1–31. Korja, A., 1995. Upper crust of the Baltic Profile, Finland. Department of Geophysics, University of Oulu, Report No. 19. 1–19. Korja, A., Elo, S., 1990. Crustal and upper mantle structure of the Wiborg batholith, SE Finland. Abstracts of the second symposium on the Baltic shield held in Lund, Sweden, June 5–7, 1990, 57. Korja, A., Heikkinen, P.J., 1995. Proterozoic extensional tectonics of the central Fenno-scandian Shield: Results from the Babel and Bothnian Echoes from the Lithosphere experiment. Tectonics 14, 504–517. Korsman, K., Koistinen, T., Kohonen, J., Wennerström, M., Ekdahl, E., Honkamo, M., Idman, H., Pekkala, Y., 1997. Bedrock map of Finland 1:1000 000. Geol. Surv. Finland, Espoo. Korsman, K., Korja, T., Pajunen, M., Virransalo, P., GGT/SVE-KA Working Group, 1999. The GGT/SVEKA Transect: structure and evolution of the crust in the Paleoproterozoic Svecofennian Orogen in Finland. Int. Geol. Rev. 41, 287–333. Kosunen, P., 1999. The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry. Bull. Geol. Soc. Finland 71, 275–304. Lahtinen, R., Nironen, M. Korja, A., 2003. Palaeoproterozoic orogenic evolution of the Fennoscandian Shield at 1.92-1.77 Ga with notes on the metallogeny of FeOx-Cu-Au, VMS, and orogenic gold deposits. In Eliopoulus et al. (Eds.), Mineral Exploration and Sustainable Development. Millpress, Rotterdam, 1057–1060. Laitakari, I., 1969. On the set of olivine diabase dikes in Häme, Finland. Bull. Comm. Géol. Finlande 241, 1–65. Laitakari, I., 1987. Hämeen subjotuninen diabaasi-juoniparvi. The Subjotnian diabase dyke swarm of Häme. In: K. Aro, I. Laitakari (Eds.), Suomen diabaasit ja muut mafiset juonikivilajit. Diabases and other mafic dyke rocks in Finland. Geol. Surv. Finland, Rep. Invest. 76, 99–116. (in Finnish with English abstract and figure and table captions) Laitakari, I., Leino, H., 1989. A new model for the emplacement of the Häme diabase dyke swarm, central Finland. In: S. Autio (Ed.), Geological Survey of Finland. Current Research 1988. Geol. Surv. Finland, Spec. Pap. 10, 7–8. Lauerma, R., 1995. Pre-Quaternary rocks of the Kursu and Salla map-sheet areas. Geological map of Finland 1:100 000, Explanation to the maps of Pre-Quaternary rocks, Sheets 3643 and 4621 + 4623. Geol. Surv. Finland, Espoo, 1–40. Liégeois, J.-P., Navez, J., Hertogen, J., Black, R., 1998. Contrasting origin of post-collisional high- K calc-alkaline and shoshonitic versus alkaline and peralkaline granitoids. The use of sliding normalization. Lithos 45, 1 – 28.
61 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Lindberg, B., Bergman, L., 1993. Vehmaan kartta-alueen kallioperä. Summary: Pre-Quaternary rocks of the Vehmaa map-sheet area. Geological map of Finland 1:100 000, Explanation to the maps of Pre-Quaternary rocks, Sheet 1042. Geol. Surv. Finland, Espoo, 1–56. Lindberg, B., Eklund, O., 1988. Interaction between basaltic and granitic magmas in a Svecofennian postorogenic granitoid intrusion, 2land, southwest Finland. Lithos 22, 13–23. Lindqvist, K., Laitakari, I., 1980. Glass and amygdules in Precambrian diabases from Orivesi, southern Finland. Bull. Geol. Soc. Finland 52, 221–229. Longhi, J., Fram, M.S., Vanderauwera, J., et al., 1993. Pressure effects, kinetics, and rheology of anorthositic and related magmas. American Mineralogist 78, 1016–1030. Mäkipää, H., 1979. Crystallization history of some postmetamorphic diabases in Åland, Häme and Satakunta, Finland. A theoretical and experimental study. Bull. Geol. Soc. Finland 51, 93–124. Marmo, J., Ojakangas R.W., 1984. Lower Proterozoic glaciogenic deposits, Eastern Finland. Geol. Soc. Am. Bull. 95: 1055-1062. Mertanen, S., Pesonen, L.J., Huhma, H., 1996. Palaeomagnetism and Sm-Nd ages of the Neoproterozoic diabase dykes in Laanila and Kautokeino, northern Fennoscandia. In: T.S. Brewer (Ed.), Precambrian Crustal Evo-lution in the North Atlantic Region. Geol. Soc. Spec. Publ. London 112, 331–358. Mitchell R.H. 1986. Kimberlites- Mineralogy, geochemistry, and petrology. Plenum, New York, 442 pp. Nironen, M., 1997. The Svecofennian orogen: a tectonic model. Precambrian Res. 86, 21–44. Paavola, J., 1980. Kallioperäkartta - Pre-Quaternary rocks, lehti - sheet 3334, Nilsiä. Suomen geologinen kartta - Geological map of Finland 1 : 100 000. Paavola, J., 1988. Lapinlahden kartta-alueen kallioperä. Summary: Pre-Quaternary rocks of the Lapinlahti map-sheet area. Suomen geologinen kartta 1:100 000, kallioperäkarttojen selitykset, lehti 3332 Lapinlahti. Geol. Surv. Finland. 1–60. Pasteris J.D. 1983. Spinel zonation in the De Beers kimberlite, South Africa: Possible role of phlogopite. Can. Mineral. 21, pp. 41-58. Patchett, P.J., Kouvo, O., Hedge, C.E., Tatsumoto, M., 1981. Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes. Contrib. Miner. Petrol. 78, 279–297. Patchett, P.J., Lehnert, K., Rehkämper, M., Sieber, G., 1994. Mantle and crustal effects on the geochemistry of Proterozoic dikes and sills in Sweden. J. Petrol. 35, 1092–1125. Pihlaja, P., 1987. Porin seudun subjotuniset dia-baasit. The Subjotnian diabases of the Pori region, southwestern Finland. K. Aro, I. Laitakari (Eds.), Suomen diabaasit ja muut mafiset juonikivilajit. Diabases and other mafic dyke rocks in Finland. Geol. Surv. Finland, Rep. Invest. 76, 133–150. (in Finnish with English abstract and figure and table captions) Peltonen, P., Kontinen, A., Huhma, H., 1996. Petrology and geochemistry of metabasalts from the 1.95 Ga Jormua Ophiolite, northeastern Finland. J. Petrol. 37, 1359-1383. Peltonen, P., Kontinen, A., Huhma, H., 1998. Petrogenesis of the mantle sequence of the Jormua Ophiolite (Finland): Melt migration in the upper mantle during Palaeoproterozoic continental break-up. J. Petrol. 39, 297-329. Peltonen, P, Mänttäri, I., Huhma, H., Kontinen, A., 2003. Archean zircons from the mantle: the Jormua ophiolite revisited. Geology 31, 645-648. Pekkarinen, L.J., 1979. The Karelian formations and their depositional basement in the Kiihtelysvaara - Värtsila area, East Finland. Geol. Surv. Finland Bull. 263, 141 p. Pihlaja, P., 1987. Laanilan diabaasi. The diabase of Laanila. Geol. Surv. Finland, Rep. Invest. 76, 189–197. (in Finnish with English abstracts and figure and table captions) Piirainen, T., 1968. Die Petrologic und die Uranlagersttten des Koli - Kaltimogebiets im finnischhen Nordkarelien. Bull. Comm Geol. Finlande, 237, 99 p.
62 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Piirainen, T., 1969. Initiaier Maginatisiiius imd seine Erzbildung in der Beleuchtung des Koli - Kaltimogebiets. Bull. Geol. Soc. Finland, 41:21- 45. Rämö, O.T., 1990. Diabase dyke swarms and silicic magmatism - Evidence from the Proterozoic of Finland. In: A.J. Parker, P.C. Rickwood, D.H. Tucker (Eds.), Mafic dykes and emplacement mechanisms. A.A. Balkema, Rotterdam–Brookfield, 185–199. Rämö, O.T., 1991. Petrogenesis of the Proterozoic rapakivi granites and related basic rocks of southeastern Fennoscandia: Nd and Pb isotopic and general geochemical constraints. Geol. Surv. Finland, Bull. 355, 1–161. Rämö, O.T., Haapala, I., 1995. One hundred years of rapakivi granite. Mineral. Petrol. 52, 129–185. Rämö, O.T., Vaasjoki, M., Mänttäri, I., Elliott, B.A., Nironen, M., 2001. Petrogenesis of the post- kinematic magmatism of the Central Finland Granitoid Complex I; radiogenic isotope constrants and implications for crustal evolution. J. Petrol. 42, 1971–1993. Rämö, O.T., Dall’Agnol, R., Macambira, M.J.B., Leite, A.A.S., de Oliveira, D.C., 2002. 1.88 Ga oxidized A-type granites of the Rio Maria region, eastern Amazonian craton, Brazil: Positively anorogenic! J. Geol. 110, 603–610. Räsänen, J., Huhma, H., 2001. U-Pb datings in the Sodankylä schist area, central Finnish Lapland. In: M. Vaasjoki (Ed.), Radiometric age determinations from Finnish Lapland and their bearing on the timing of Precambrian volcano-sedimentary sequences. Geol. Surv. Finland, Spec. Pap. 33, 153–188. Rastas, P., Huhma, H., Hanski, E., Lehtonen, M.I., Härkönen, I., Kortelainen, V., Mänttäri, I., Paakkola, J., 2001. U-Pb isotopic studies on the Kittilä greenstone area, central Lapland, Finland. In: M. Vaasjoki (Ed.), Radiometric age determinations from Finnish Lapland and their bearing on the timing of Precambrian volcano-sedimentary sequences. Geol. Surv. Finland, Spec. Pap. 33, 95–141. Rautiainen, J., 2000. Arkeeisen kratonin reunalla esiintyvät intermediääriset juonet Iisalmen, Juankosken ja Siilinjärven alueilla. M.Sc. thesis, Department of Geology, University of Helsinki. (in Finnish) Reid A.M., Donaldson C.H., Dawson J.B., Brown R.W., Ridley W.I. 1975. The Igwisi hills extrusive "kimberlites". Phys. Chem. Earth 9, pp. 199-218. Rutanen, H., 2001. Geochemistry and petrogenesis of the 1.8 Ga old post-collisional intrusions in southern Finland and Russian Karelia. Lic.Phil. Thesis, Department of Geology and Mineralogy, 2bo Akademi University, Finland. Rutanen, H., Eklund, O., Konopelko, D., 1997. Rock and mineral analyses of Svecofennian postorogenic 1.8 Ga intrusions in southern Finland and Russian Karelia. Geocenter Raportti—Geocenter Rapport, vol. 15. Turku University-2bo Akademi University. Saini-Eidukat, B., Alapieti, T.T., Thalhammer, O.A.R., Iljina, M.J., 1997. Siliceous, highmagnesian parental magma compositions of the PGE-rich Early Paleoproterozoic layered intrusion belt of northern Finland. In: P. Rongfu, (Ed.), Proceedings of the 30th International Geological Congress, Beijing, China, 4–14 August 1996. Vol. 9: Energy and Mineral Resources for the 21st Century, Geology of mineral deposits, mineral economics. VSP, Utrecht, 177–197. Salonsaari, P.T., 1995. Hybridization in the subvolcanic Jaala-Iitti complex and its petrogenetic relation to rapakivi granites and associated mafic rocks of southeastern Finland. Bull. Geol. Soc. Finland 67 (1b), 1–104. Shebanov, A.D., Eklund, O., 1997. Interaction between acid and basite magmas in the Hammarudda Complex, SW Finland: P-T conditions obtained by mineralogical thermometry and barometry. Petrol. 5, 141–166. Siivola, J., 1987. Lovasjärven mafinen intruusio. The mafic intrusion of Lovasjärvi. In: K. Aro, I. Laitakari (Eds.), Suomen diabaasit ja muut mafiset juonikivilajit. Diabases and other
63 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
mafic dyke rocks in Finland. Geol. Surv. Finland, Rep. Invest. 76, 121–128. (in Finnish with English abstract and figure and table captions) Sipilä, P., 1992. The Caledonian Halti-Ridnitsohkka igneous complex in Lapland . Geol. Surv. Finland, Bull. 362, 1–75. Skyttä, P., 2002. Emplacement of the Åva ring intrusion: some structural and magmatic aspects. M.Sc. Thesis, Department of Geology, University of Turku, Finland. Suominen, V., 1987. Lounais-Suomen mafiset juonikivet. Mafic dyke rocks in southwestern Finland. In: K. Aro, I. Laitakari (Eds.), Suomen diabaasit ja muut mafiset juonikivilajit. Diabases and other mafic dyke rocks in Finland. Geol. Surv. Finland, Rep. Invest. 76, 151–172. (in Finnish with English abstract and figure and table captions) Suominen, V., 1991. The chronostratigraphy of southwestern Finland with special reference to Postjotnian and Subjotnian diabases. Geol. Surv. Finland, Bull. 356, 1–100. Toivola, V., 1988. Sonkajärven - Varpaisjärven alueen diabaasit. Pro gradu-tutkielma, Helsingin yliopisto, 73s. Toivola, V., Huhma, H., Paavola, J., 1991. The dolerite dykes in the Sonkajärvi – Varpaisjärvi area, Central Finland. Geol. Surv. Finland, Spec. Pap. 12, 59-62. Törnroos, R., 1984. Petrography, mineral chemistry and petrochemistry of granite porphyry dykes from Sibbo, southern Finland. Geol. Surv. Finland, Bull. 326, 1–43. Turchenko, S.I., Semenov, V.S., Amelin, Ju.V., Levchenkov, O.A., Neymark, L.A., Buiko, A.K., Koptev–Dvornikov, Je.V., 1991. The Early Proterozoic riftogenic belt of Norhern Karelia and associated Cu-Ni, PGE and Cu-Au mineralizations. Geol. För. Stockholm. Förhandl. 113, 70–72. Tyni, M. 1997. Diamond prospecting in Finland – a review, In Papunen, H., ed., Mineral Deposits: Research and Exploration, where do they meet?, Balkema, Rotterdam. pp. 789–791. Vaasjoki, M., 1996. Explanation to the geochronological map of southern Finland: The development of the continental crust with special reference to the Svecofennian orogeny. Geol. Surv. Finland, Rep. Invest. 135, 1–30. Vaasjoki, M., Sakko, M., 1988. The radiometric age of the Virmaila diabase dyke: evidence for 20 Ma of continental rifting in Padasjoki, southern Finland. Geol. Surv. Finland, Spec. Pap. 10, 43–44. Vaasjoki, M., Rämö, O.T., Sakko, M., 1991. New U-Pb ages from the Wiborg rapakivi area: constraints on the temporal evolution of the rapakivi granite–anorthosite–diabase dyke association of southeastern Finland. In: I. Haapala, K.C. Condie (Eds.), Precambrian granitoids. Petrogenesis, geochemistry and metallogeny. Special Issue, Precambrian Res. 51, 227–243. Väisänen, M., Hölttä, P., 1999. Structural and metamorphic evolution of the Turku migmatite complex, southwestern Finland. Bull. Geol. Soc. Finland 71, 177– 218. Väisänen, M., Mänttäri, I., 2002. 1.90–1.88 Ga arc and back-arc basin in the Orijärvi area, SW Finland. Bull. Geol. Soc. Finland 74, 185– 214. Väisänen, M., Mänttäri, I., Kriegsman, L.M., Hölttä, P., 2000. Tectonic setting of post-collisional magmatism in the Paleoproterozoic Svecofennian orogen, SW Finland. Lithos 54, 63– 81. Vuollo, J., 1988. Kolin kerrosjuonen petrologia- Kaunisniemen profiili. Pohjois-Karjalan malmiprojekti, Oulun yliopisto. Raportti No 16, 122 p. Vuollo, J., 1994. Palaeoproterozoic basic igneous events in Eastern Fennoscandian Shield between 2.45 Ga and 1.97 Ga, studied by means of mafic dyke swarms and ophiolites in Finland. Acta Univ. Ouluensis Ser. A 250, 1–47. Vuollo, J., Huhma, H. 2004. Radiometric age determinations from mafic intrusions, dykes and their bearing on the timing of Precambrian mafic magmatic events. Geol. Surv. Finland, Spec. Pap. (in prep.)
64 FIFTH INTERNATIONAL DYKE CONFERENCE 31.7 – 3.8.2005 – ROVANIEMI, FINLAND
Vuollo, J., Piirainen, T., 1989. Mineralogical evidence for an ophiolite from Outokumpu serpentinites in North Karelia, Finland. Bull. Geol. Soc. Finland, 61:l Vuollo, J., Piirainen, T. 1992. The 2.2 Ga old Koli layered sill : the low-Al tholeiitic (karjalitic) magma type and its differentiation in northern Karelia, eastern Finland. Geol. För. Stockholm Förhandl. 114 (1), 131-142. Vuollo, J., Huhma, H., Pesonen, L.J., 2000. Mafic dyke swarms – geological evolution of the Palaeoproterozoic in the Fennoscandian Shield. In: L.J. Pesonen, A. Korja, S.-E. Hjelt (Eds.), Lithosphere 2000 - A symposium on the structure, composition and evolution of the lithosphere in Finland. Programme and extended abstracts, Espoo, Finland, October 4–5, 2000. Institute of Seismology, University of Helsinki. Report S–41, 107–111. Vuollo, J., Piirainen, T., Huhma, H., 1992. Two Early Proterozoic tholeiitic diabase dyke swarms in the Koli-Kaltamo area, eastern Finland. Geol. Surv. Finland. Bull. 363. 32p
65