Geological Survey of Finland Bulletin 357 Paleoproterozoic volcanism in the Kühtelysvaara - Tohmajärvi district, eastern Finland by Lauri J. Pekkarinen and Heikki Lukkarinen Geologian tutkimuskeskus Espoo 1991 - - ----- --- - - Geological Survey of Finland, Bulletin 357 P ALEOPROTEROZOIC VOLCANISM IN THE KIIHTELYSVAARA - TOHMAJÄRVI DISTRICT, EASTERN FINLAND by LAURI J. PEKKARINEN AND HEIKKI LUKKARINEN with 17 figures, 1 table and 3 appendices GEOLOGIAN TUTKIMUSKESKUS ESPOO 1991 Pekkarinen, L.J. & Lukkarinen, H., 1991. Paleoproterozoie volcanism in the Kiih­ telysvaara - Tohmajärvi distriet, eastern Finland. Geological Survey 0/ Finland, Bulletin 357,30 pages, 17 figures, I table and 3 appendiees. The numerous episodes of Paleoproterozoie volcanism and assoeiated sedimen­ tation preserved in the Kiihtelysvaara - Tohmajärvi distriet represent both prolonged and episodie rifting of eratonie erust. V-Pb zireon age determinations from two mafie dykes intruded in eonjunetion with the basalmost lava flows indieate an age of ca. 2120 - 2100 Ma for this earliest mafie magmatism. However, no informa­ tion is yet available eoneerning the age of the younger flows, dykes or sills. Pyroclastie units are also present and have, along with the mafie lava flows and intrusions, been metamorphosed under greenschist facies eonditions. Volcan­ ism took plaee in an intraeratonie, within-plate setting, with predominantly basal­ tie eompositions. Hydrothermal alteration oeeurred both during and after erup­ tion. This affeeted the chemical composition of the earlier lava in particular, the younger lava flows as weil as the dykes and sills generally showing less evidence of ehemical alteration. The volcanogenic and sedimentary formations of the Kiihtelysvaara area have been renamed according to local geographical place names and regional correla­ tions and comparisons have been made between the study area and similar sequences elsewhere in Finland, particularly with respect to the mafie units. Key words: metavolcanic rocks, metabasite, hydrothermal alteration, rifting, absolute age, stratigraphy, correlation, Paleoproterozoic, Proterozoic, Kiihtelys­ vaara, Tohmajärvi, North Karelia, eastern Finland Pekkarinen L.J., Outokumpu Finnmines Oy, Exploration, P.O. Box 67, SF-83501 Outokumpu, Finland Lukkarinen H., Geological Survey 0/ Finland, P. O. Box 1237, SF-70701, Kuopio, Finland ISBN 951-690-446-7 ISSN 0367-522X Vammalan Kirjapaino Oy 1991 CONTENTS Introduction .... 5 General geology and stratigraphy ...... ......... ........ .. .. .. 7 Metabasite petrography . ................. ....... .. .. ............ 10 Older metabasites .. ....... ........ .. ...... .. ............. .. .. 10 Y ounger metabasites . .. ... ....... ...... .... ........... .. 10 Metabasite geochemistry . .. ...... ...... ...... .. .... .. .... 11 Sampling and analytical methods . ..... ..... .................. 11 Alteration and effects on whole-rock geochemistry . 11 Major element geochemistry ... 13 Trace element geochemistry ....... ..................... .. ... 16 Radiometrie ages of metabasites ............... ................ ...... 19 Disdussion ............ .... .. ........... ......... .... .. 19 Alteration and petrogenesis ............ .. ...................... 19 Tectonic setting ..... .. .... ... .... .. .. .................. 22 Dyke rocks in the Archaean craton .... .. .. ... .......... ..... 24 Correlation .. ... 24 Conciuding remarks ....... .. .......... .. .................... .. 26 Acknowledgments ........... .... .................................. 26 References .............. .... .. .. ... ... ..... .. ..... .. .. 27 Appendices: 1. Major (wt-OJo) and trace (ppm) element compositions of metabasites from the Kiihtelysvaara - Tohmajärvi district 2. Sampling sites and short descriptions of sampies 3. Sampling sites for Nd-Sm and U/ Pb-age measurements and short descriptions of sampies INTRODUCTION The Archaean basement gneiss complex in (1971 a) nevertheless considered that the meta­ North Karelia is bounded in the west by a se­ diabase dykes cutting the basement gneis ses cor­ quence of Paleoproterozoic sediments and vol­ respond to Fe-rich tholeiites, whereas lava flows canogenic lithologies. In addition to lavas, within the Proterozoic sequence apparently be­ pyroclastic deposits and tuffaceous sediments, long to the spilite-keratophyre association. the metavolcanic sequence includes metadiabase The purpose of this study is to describe the dykes and sills (Nykänen, 1971a,b; Piirainen petrography, stratigraphy and chemical charac­ et al. , 1974; Pekkarinen, 1979). This so-called teristics of the metavolcanites and related intru­ Jatulian phase of volcanism took place between sives in the Kiihtelysvaara - Tohmajärvi districL 2200-2000 Ma (Sakko, 1971; Kouvo, 1976; The stratigraphieal scheme of Pekkarinen (1979) Simonen, 1980; Meriläinen 1980a,b; Huhma, has been modified somewhat, such that forma­ 1986a). Piirainen (1968) divided the Jatulian mafie tions and groups (Figure 3) have been named dykes intruding both basement gneisses and after geographieal place names (cf. Hedberg, cover quartzites in the Koli area into two sepa­ 1976). It is suggested that the terms Sariola, Jatuli rate types - a 'drier', tholeiitic association and and Kaleva (cf. Meriläinen, 1980a,b; Luukkonen & a more volatile-rich spilitic group. According to Lukkarinen, 1986) be used with reference to tec­ Hanski (1986b), the earlier (ca. 2200 Ma) Jatulian tofacies rat her than as formallithostratigraphi­ intrusions constitute a distinct gabbro-wehrlite cal units (cf. Laajoki, 1988). Because all volcano­ association which represents a more primitive genic lithologies and dykes in the study area have parent magma than that of later magmatism. been to some extent metamorphosed and are in The geology of the Kiihtelysvaara - Toh­ many instances intensely deformed, they may col­ majärvi region is comparatively weil known, lectively be referred to as metabasites. However, having been described earlier in some detail by, for convenience the prefix meta- will not be used among others, Nykänen (1968, 1971a,b), Pekka­ here further . rinen (1979) and Ward (1987, 1988). However, Both authors have participated in interpreta­ there has been rather less information available tion and preparation of the manuscripL Lukka­ concerning the geochemistry of the metavol­ rinen however was responsible for producing the canites and associated mafic dykes (cf. Nykänen, chemical variation and discrimination diagrams, 1968, 1971a; Pekkarinen, 1979). Likewise, their while Pekkarinen summarized these results, de­ origin, tectonie setting and alteration system­ veloped a model for describing alteration effects atics have not been examined in detail. Nykänen and revised the stratigraphical terminology. 6 Geologieal Survey of Finland, Bulletin 357 A '. USSR '\ N \ \. '\ '-. " Bas"m"nl \ , '\ '\ '\ \ ) '\ ' 11 ;' \\ 11 .•1)... 1\ / 40 km \ 11 ~ Iloma~isl i ~ ~- ~---- +,"A"r,,- - 11 ,\'\ ~~ l~telysvoa ra ;'1\ ~ Formations of ~/~ the Viesimo, Raatevaara and " ' Hyypiä Groups r=:-=l Formations of the "=-=-=J Tohmajärvi Group Metadiabase dykes and sills Area In Fig B r-­ I Area In Fig. 3 I EARLY PROTEROZOIC FORMATIONS o 2km HelnQvoarQ end Koljola Fm ~ Kartel/aara Fm ~' . *":' ARCHAEAN BASEMENT EJ Dttala Fm D Haukilampi Fm Vltstola end Sarkllampl end Migmatites, gneisses ~ Petolkko Fm ~o 0 Polkkylampl Fm I-",~ 1and granitaids Mptadlobase dyke Kalkunmakl Fm Metapelites and mafie 0- - \" Metadlobase sill IlIlIj volcanic rocks Fig. I. Stratigraphie map of North Karelia (A) modified after Luukkonen ja Lukkarinen (1986) and (B) simplified geologi­ eal map of the Hyypiänvaara - Raatevaara area. Line A-B eorresponds LO seetion in Fig. 10. Numbers refer LO sites sampled for analyses. Geological Survey of Finland, Bulletin 357 7 GENERAL GEOLOGY AND STRA TlGRAPHY The Kiihtelysvaara - Tohmajärvi district is 10- elast conglomeratic intercalations in its lower cated in eastern Finland, along the boundary part. Both the Viesimo and Raatevaara Groups zone between the Archaean basement complex have been intruded by numerous mafic dykes. and the Proterozoic Karelidic schist belt (Sirno­ Volcanic activity in the Kiihtelysvaara district nen, 1980). The Archaean craton (Figure 1) con­ commenced with the eruption of mafie lavas onto sists of greenstone belts and granitoids that Haukilampi Formation quartzites. The latter formed between 2.9-2.5 Ga (Wetherill et al., were evidently already consolidated by this stage, 1962; Kouvo & Tilton, 1966; Tilton & Grünen• since lava has been found occupying fissures in felder, 1968; Nykänen, 1968, 1971a,b; Simonen, the quartzites and conversely, quartzite enelaves 1980; Proskuryakov, 1983; Lavikainen, 1986, occur within the lavas (Nykänen, 1971a). The 1989; Ward & Nurmi, 1989). Paleoproterozoic mafic sequence, designated as the Koljola For­ mafic dykes and dyke swarms are very abundant mation, varies from 30-80 m in thickness and within the craton (Nykänen, 1968, 1971a,b; Pek­ consists of numerous lava flows as weIl as, in the karinen, 1979; Lavikainen, 1986), typically oc­ uppermost part, pyroclastic deposits and tuffa­ curring with a frequency of 2-3 dykes/ km' ceous sediments. The formation can be traced, (Pekkarinen, unpublished data). Those dykes at the present erosion level, from north of the vil­ causing magnetic anomalies are evident from the lage of Kiihtelysvaara to south as far as the Finn­ gray-tone aeromagnetic pixel map shown in Fig­ ish - Soviet border. The mafic volcanic rocks ure 2. In general the dykes