Aegirine and Riebeckite of the Alkali Gneiss of Pikkukallio in the Honkamäki—Otanmäki Region, Finland
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AEGIRINE AND RIEBECKITE OF THE ALKALI GNEISS OF PIKKUKALLIO IN THE HONKAMÄKI—OTANMÄKI REGION, FINLAND KAI HYTÖNEN and TUULA HAUTALA HYTÖNEN, KAI and HAUTALA, TUULA, 1985: Aegirine and riebeckite of the alkali gneiss of Pikkukallio in the Honkamäki—Otanmäki region, Finland. Bull. Geol. Soc. Finland 57, Part 1—2, 169—180. Aegirine and riebeckite coexist in a Precambrian peralkaline gneiss. Chemical analyses of the gneiss and chemical and physical data on the aegirine and the rie- beckite are presented. The compositions of the two minerals are compared with those of other sodic pyroxenes and alkali amphiboles from Finland. The authors know of only two other occurrences in Finland where sodic pyroxene and alkali amphibole coexist. The riebeckite of the present study and the ferro—ferri- barroisite of the adjoining Otanmäki are richer in iron and poorer in magnesium and calcium than the other known alkali amphiboles and sodic-calcic amphiboles from Finland. Zircon age of 2019 Ma has been obtained for a variety of the alkali gneiss of the region. Key words: aegirine, riebeckite, chemical data, x-ray data, alkali gneiss, zircon age, Finland. Kai Hytönen and Tuula Hautala: Geological Survey, SF—02150 Espoo, Finland. Introduction for the X-ray work, optical data and writing, and Hautala for the microprobe analyses. The alkali gneiss of this study has been de- scribed briefly in a paper on the granites of Honkamäki and Otanmäki by Marmo et al. Material and methods (1966, cf. pp. 12—13 and 15, Table 2, photo- micrographs Fig. 9 and Plate 1). The paper in- The subject of the present study is a specimen cluded an analysis of the rock, which was called collected in 1963, and numbered 7—OM for the alkali granite, and some optical data on the study by Marmo et al. (op. cit.), from the main femic minerals, aegirine and riebeckite. northern slope of the Honkamäki hill, which is Aegirine and riebeckite have since been anal- situated about 8 km west-northwest of the ysed and studied by X-ray diffraction, and Otanmäki titaniferous iron ore deposit and some trace elements of the rock determined. mine (Fig. 1). Named Pikkukallio (Fig. 2) on The present paper gives the results of these the topographic map, the outcrop has the studies. Of the authors, Hytönen is responsible following coordinates: map sheet 3413 11, 170 Kai Hytönen and Tuula Hautala x = 7115.75, y = 497.40 or 64°08'28" North, dium). The measured intensities were corrected 26°56'48" East. using the Bence and Albee (1968) method. The X-ray powder data were obtained with a Philips diffractometer, filtered Copper radia- tion. Silicon was used as the internal standard Alkali gneiss for aegirine and quartz for riebeckite. The microprobe analyses were conducted in 1975 The rock of the Honkamäki hill in the Otan- using a Geoscan electron probe analyser with an mäki region (Figs. 1 and 2) in the parish of Vuo- accelerating potential of 20 kV and a specimen lijoki already drew some attention during the current of 3—4 mA. The standards were horn- mapping of the general geological map of Fin- blende (no. 35) and oligoclase (no. 44, for so- land. On map sheet C4 Kajaani (Wilkman 20 STRIPED GNEISS] PRESVEKO- TITANIFEROUS IRON ORE I J 'L\ ALKALIC GNEISS > KARELIAN HETEROGENOUS ZONE CONSISTING BASEMENT AMPHIBOLITE OF ANORTHOSITE-ANORTHOSITE - OTANMAKI 3 MICROCLINE GRANITE GABBRO AND ORTHO AMPHIBOLITE ASSOCIATION SUPRACRUSTAL ROCKS GABBRO FAULT ORTHOAMPHIBOLITE-METAGABBRO Fig. 1. Geological map of the Otanmäki region, according to Lindholm and Anttonen 1980. 1. Alkali gneiss of the present study (specimen 7—OM), containing aegirine and riebeckite. 9. Ferro-ferri-barroisite occurring as fissure fillings in alkali gneiss at the western foot of the Otanmäki hill (Hytönen and Heikkinen 1966). Aegirine and riebeckite of the alkali gneiss of Pikkukallio in the Honkamäki-Otanmäki region, Finland 171 t97 Table 1. Alkali gneiss, 7—OM, Pikkukallio, Honkamäki, Salonsuo Vuolijoki. Chemical analysis, CIPW norm, Niggli numbers and modal composition. Hautakangas pikkukaUi, Wet analysis by CIPW norm Mode, wt. Aulis Heikkinen Point counting, X wt. % 1690 points /Specimen 7-orr wt. % Quartz 14 Si02 64.48 Q 13.0 Ti02 1.11 z 0.1 Microcline 39 19 AI2O3 12.20 or 30.3 Albite 34.2 7115. Fe203 6.00 ab Salic 72 hl 0.1 HONKAMAKI FeO 3.00 Aegirine 12 A 200,3 m MnO 0.29 Salic 77.7 MgO 0.29 Riebeckite 10 CaO 0.40 ac 12.2 Opaque 4 0.9 Access. 2 Na20 5.68 di monazite K26 5.13 hy 3.0 mt 2.6 titanite P2O, 0.04 co2 0.00 il 2.1 zircon epidote Zr02 0.08* ap 0.1 + H2O 0.51 fr 0.2 mica fluorite H2O- 0.06 Femic 21.1 F 0.09 Femic 28 CI' 0.04 99.40 Niggli numbers -0 = F,, si 267.6 0.05 CU al 29.8 99.35 fm 32.0 c 1.8 * XRF analysis by Maija Hagel- alk 36.4 Fig. 2. Location of the alkali gneiss outcrop of Pikkukallio Brunnström and Väinö Hoffrén k 0.37 (cross, specimen 7—OM) on the northern slope of the Hon- mg 0.06 kamäki hill. c/fm 0.06 1929), the rock, which is predominant in large heterogeneous rock unit with metasomatic fea- parts of the Vuolijoki parish, including Honka- tures». mäki, is designated post-Bothnian granite with In the present study the name alkali gneiss is inclusions. In the explanation to the map sheet used for compositional and textural reasons. If (Wilkman 1931, p. 148) the rock occurring on the rock is regarded as plutonic, the name the northern slope of the Honkamäki hill is microcline-albite quartz syenite should be used mentioned as »a reddish hornblende granite» (Streckeisen 1974). The alkali gneiss is fine- to containing darker gneissic hornblende-rich medium-grained (grain size 0.3—2.0 mm) with bands. distinct planar banding caused by alternating The same rock was later studied by Pääkkö- dark femic and reddish felsic streaks. Under the nen (1956, cf. pp. 19—22), who called it gneis- microscope the texture is weakly hypidiomorphic, sose granite and, as mentioned above, by Mar- in places slightly cataclastic, and the banding is mo et al. (op. cit.). In a fairly recent study of only slightly detectable. The main minerals of the geology of the Otanmäki mine by Lindholm the rock are microcline, albite, quartz, aegirine and Anttonen (1980), the same rock is termed and riebeckite (Table 1). The microcline is alkalic gneiss and is characterized as a »very cross-hatched (triclinicity 0.9) and perthitic. 172 Kai Hytönen and Tuula Hautala The albite is usually twinned, and some anti- Examination of Table 1 reveals that there is perthite can be seen. Both feldspars contain a difference between normative or and modal abundant opaque inclusions. Quartz has un- microcline, between normative ab and modal dulatory extinction. The femic constituents albite and between the sum of the normative usually occur as irregularly shaped crystal ag- femic silicates (ac + di + hy = 16) and the sum of gregates, arranged into irregular bands. Among aegirine + riebeckite (22), which should ap- the accessories, monazite forms quite large proximately correspond with each other. The grains, approximately 0.2 mm long. A qualita- difference can be attributed partly to the dif- tive microprobe analysis of the monazite gave: ference in composition between the sample Ce, La, Nd, Pr, Sm (?), Gd and P. analysed (norm) and the thin section studied Table 1 gives the chemical analysis, CIPW norm, mode and Niggli values of the alkali gneiss. The peralkaline character is portrayed by the presence of acmite, NaFeSi206, in the Table 3. Aegirine, specimen 7—OM, Pikkukallio, Honka- norm. mäki, Vuolijoki. Chemical composition, numbers of ions on the basis of six oxygens, pyroxene end member composi- tion and calculated densitity. 1. Average of four microprobe analyses (FeO = total iron); 2. Analysis of column 1, done using the ratio Fe2+: Fe3 + =0.19 (wet analysis); 3. Analysis Table 2. Comparison of some trace element contents (ppm) of column 1, done using the values of Fe203 and FeO ob- of the alkali gneiss, 7—OM, Pikkukallio, Honkamäki, tained in the wet analysis. Wet analyses by Pentti Ojanperä. Vuolijoki with those of the granite averages of Taylor (1964) and felsic rocks (granites, granodiorites, etc.) of 1 2 3 Vinogradov (1962). Si02 50.8 50.8 50.8 Alkali gneiss Taylor Vinogradov Ti02 0.3 0.3 0.3 7- OM AI2O3 1.3 1.3 1.3 29.1 27.1 1 Fe203 Li 36 30 40 FeO 31.2 5.0 4.6 2 Sc <25 5 3 0.5 2 MnO 0.5 0.5 V < 13 20 40 MgO 0.3 0.3 0.3 2 Ni <15 0.5 8 CaO 3.3 3.3 3.3 Cu < 152 10 20 Na20 11.5 11.5 11.5 Zn 340' 40 60 K2O 0.1 0.1 0.1 Ga 602 18 20 Rb 3003 150 200 99.3 102.2 99.8 Sr 16' 285 300 3 Si 1.94 Y 90 40 34 >2.00 Zr 6203 180 200 AL 0.06 0.03' Nb no3 20 20 AL 0.03 2 1 Ti 0.01 0.01 Mo <10 2 3 + 2 830 Fe 0.84 0.79 Ba ClOO 600 2+ •1.02 La 191" 40 60 Fe 0.16 1.05 0.15 Ce 315" 100 Mn 0.02 0.02 Hf 22.94 4 1 Mg 0.02 0.02 J 0.141 Ta 11.7" 3.5 3.5 Ca 0.13 Pb 52 20 20 Na 0.85 (0.99 0.86 >1.01 Th 29.3" 17 18 K 0.01 0.01 4 U 4.5 4.8 3.5 acmite NaFe3 + 84 81 Th/U 6.5 jadeite NaAl — 3 diopside CaMg2+ 2 2 1 AAS analysis by Meeri Wallgren hedenbergite CaFe2+ 10 10 2 Optical spectrometric analysis by Ari Puisto ferrosilite Fe2+Fe3+ 4 4 3 XRF analysis by Maija Hagel-Brunnström and Väinö Hoffrén 100 100 4 Instrumental neutron activation analysis by Rolf Rosen- D calculated 3.61 3.59 berg (Reactor Laboratory, Technical Research Centre of Finland) Aegirine and riebeckite of the alkali gneiss of Pikkukallio in the Honkamäki-Otanmäki region, Finland 173 (mode), and partly to the fact that for the 3).