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Mafic and Ultramafic Igneous Rocks of the Raisduoddar- Halti Area in the Finnish-Norwegian Caledonides

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Mafic and Ultramafic Igneous Rocks of the Raisduoddar- Halti Area in the Finnish-Norwegian Caledonides MAFIC AND ULTRAMAFIC IGNEOUS ROCKS OF THE RAISDUODDAR- HALTI AREA IN THE FINNISH-NORWEGIAN CALEDONIDES. PETROGRAPHY, MINERALOGY AND GEOCHEMISTRY PEKKA SIPILÄ SIPILÄ, PEKKA, 1991: Mafic and ultramafic igneous rocks of the Raisduoddar- Halti area in the Finnish-Norwegian caledonides. Petrography, mineralogy and geochemistry. Bull. Geo!. Soc. Finland 63, Part 1, 15—24. The mafic and ultramafic rocks in the Raisduoddar-Halti area constitute a shal- low horizontal allochthonous plate in the highest tectono-stratigraphic position of the Finnish Caledonides. They are located close to the boundary of the lower part of the Vaddas Nappe in the Upper Allochthon and the Kalak Nappe Complex in the Middle Allochthon. The mafic and ultramafic rocks in the area are Ridnitsohk- ka gabbro sills in the east and dunite-troctolite-olivine gabbro cumulates at Rais- duoddar-Halti. They are syngenetic and belong to the same nappe. The lower con- tact with the Nabar schists of the Kalak Nappe Complex on the cumulate side is distinctly tectonic and may continue as a tectonic contact in the sill area, too. The gabbro sills, which are tholeiitic in chemical composition, are enriched in LREE and show a small positive Eu anomaly. Close to the cumulate the gabbro sills contain olivine. The cumulate massif crystallized mainly as a closed system, as shown by the linear change in the composition of the rocks and minerals. The cumulates are in- truded by coarse grained gabbro pegmatoids. The same pegmatoid magma gave rise to the metasomatic olivine gabbro with augite porphyroblasts between the cu- mulate massif and the sill area. Before last nappe movements the rocks of the cu- mulate massif were partly amphibolitized across the lithologic contacts and mag- matic layering. As suggested by the total lithologic composition and the mineral composition, the magmatism in the area contains evidence of at least two major magmatic events. First, tholeiitic gabbro sills intruded the acid schists. Then, the magma type having become more primitive the Raisduoddar-Halti cumulates fractionated. A new U-Pb age of 434 + 4 Ma on baddeleyite from Ridnitsohkka gabbro sills shows that the mafic and ultramafic rocks in the area can be correlated with the synorogenic Scandian mafic intrusive rocks in the Scandinavian Caledonides. Key words: gabbros, sills, cumulates, chemical composition, electron probe data, caledonides, Paleozoic, Ridnitsohkk"a, Enontekiö, Finland. Pekka Sipilä: Department of Geology, University of Turku, SF-20500 Turku, Fin- land. Present address: Geological Survey of Finland, SF-02150, Espoo, Finland. Introduction parts of the country, between Sweden and Nor- way (Fig. 1, Index map). Detailed information The Scandinavian Caledonides extend to Fin- about the geology of the Finnish Caledonides has land only within a small area in the northernmost been obtained only recently as a result of regional 16 Pekka Sipilä RAISDUODDAR-HALTI NORWAY FINLAND 10 km YZÅ UPPER ALLÜCHTHON MIDDLE ALLÜCHTHON CALEDONIAN LOWER ALLOCHTHON AUTOCHTHON —7—7 7 N S S / / / N \ \ ARCHEAN BASEMENT Fig. 1. The Caledonides in Finland. Modified from Lehtovaara 1986. mapping undertaken there by geologists from the chaean basement. The ultramafic and mafic University of Turku. The mafic and ultramafic rocks in the Raisduoddar-Halti area are close to rocks in the area have been studied by Hausen the boundary of the Lower Vaddas Nappe and (1941, 1942), Böe (1976), Lehtovaara and Sipilä Upper Kalak Nappe and they occupy the highest (1987) and Sipilä (1987, 1988, 1989). tectono-stratigraphic and topographic position in The greater part of the Finnish Caledonides be- the Finnish Caledonides (Lehtovaara 1986, 1989). longs to the Kalak Nappe of the Middle Alloch- The position of the Raisduoddar-Halti is interest- thon (Fig. 1). The autochthonous cover and the ing tectonostratigraphically as it is located in the Lower Allochthon, which are exposed as a nar- debated boundary zone between the Finnmarki- row rim in the southeastern margin of the an and Scandian phases of the Caledonian oroge- Caledonian overthrust nappes, rest on the Ar- ny. The Finnish Caledonides have usually been Mafic and ultramafic igneous rocks of the Raisduoddar-Halti area in the 17 considered to belong to the allochthons of the thopyroxene. Close to the cumulate massif, the Finnmarkian phase (Zwaan and Roberts 1978). gabbro sills contain olivine. In places, pyroxene Since the nearest Scandian phase allochthon is is intensely uralitized and kelyphytic rims have only a few kilometers away, it has also been sug- developed between olivine and plagioclase. The gested that the rocks at Raisduoddar-Halti may modal composition of the gabbro sills is shown belong to the lower parts of Scandian nappes in Fig. 3. Olivine is nearly totally altered and it (Lehtovaara 1984). Recent data on U-Pb ages is represented by the alteration products in the from Ridnitsohkka gabbro sills (Vaasjoki et al., mode. The gabbro sills show weakly developed in prep.) show that the Scandian phase alloch- layering close to the cumulate massif. There are thons extend to Finland. The classification of the no cooling contacts with the gneiss interlayers, rocks into the orogenic phases in the northern rather they were partly melted. Norway is, however, a controversial subject, and some researchers have questioned the existence The Raisduoddar-Halti cumulate massif of the Finnmarkian phase (Krill and Zwaan 1987). The cumulates at Raisduoddar-Halti are ex- The Raisduoddar-Halti Magmatic Complex posed within an area of about 14 km2. Further- occurs as a horizontal allochthonous sheet on the more, in the west there are three small satellites summits of the Raisduoddar-Halti and Ridnit- separated by erosion. The bulk of the massif is sohkka fells on both sides of the Finnish- composed of dunite-troctolite-olivine gabbro cu- Norwegian border (Fig. 2). The complex is com- mulates. The troctolite and olivine gabbro exhibit posed of Ridnitsohkka gabbro sills ( = dolerite distinct layering, which in the east is subvertical dyke swarm by Böe, 1976) in the east and Rais- but in the northeast and west dip gently towards duoddar-Halti dunite-troctolite-olivine gabbro the centre of the massif. Synmagmatic breccia cumulates. The rocks overlying the sheet have structures show that accumulation took place in been eroded. The lower contact with the Nabar a tectonically unsettled environment. The brec- schists is distinctly tectonic on the side of the cu- cia matrix is heterogeneous, varying from peg- mulates. The eastern margin of the sill area is sur- matoidal olivine gabbro to troctolitic dunite with rounded by a agmatitic garnet-kyanite contact small amounts of plagioclase. A breccia may con- aureole approximately 500 m thick. tain both dunitic and anorthositic fragments, of which the largest measure several meters across. The brecciating material was often crystal mush, Petrography as demonstrated by the plagioclase laths orient- ed concordantly with the breccia fragments. The The Ridnitsohkka gabbro sills modal composition of the cumulate massif is The eastern Ridnitsohkka sill area is about 2 shown in Fig. 3. km wide. It extends northward following the In the eastern part of the massif there is a co- shape of the cumulate massif. The conformable herent northwards tapering dunite tongue with garnet-sillimanite gneiss interlayers are up to fifty narrow discontinuous troctolite and anorthosite meters wide and dip invariably towards the cu- interlayers and a few stratiform chromitite lay- mulates. The frequency of the gneiss interlayers ers. In general, the dunite is massive in texture. is highest farthest from the cumulate massif. In Initially it was coarse-grained, but later the oli- chemical composition, the gabbro sills are vine grains were intensely mechanically granulat- tholeiitic; they are enriched in LREE and show ed. Chrome spinel is a common accessory. At a small positive Eu anomaly. The main primary their thickest, the discontinuous chrome spirel minerals are plagioclase, clinopyroxene and or- layers measure 20 cm. 2 Raisduoddar- Halti Cumulates : RlDNITSOHKKA GABBRO SILLS -i DUNITE Bedding S with Sill-Gar-gneiss interlayers Foliation TROCTOLITE Nabar Nappe (Upper part of Kalak Nappe Complex) Shear -e- OLIVINE GABBRO Augite augen k ^^ Fault Kyanite AMPHI BOLE ROCK Eastern boundary of Gabbro Sills Thrust Fig. 2. Geological map of Raisduoddar-Halti and Ridnitsohkka areas. Mafic and ultramafic igneous rocks of the Raisduoddar-Halti area in the ... 19 "90 OLIVINE "80 FO (mol®) 1 IT CHROME SPINEL h0.8 chrome spinel AL/AL+CR +FE3+ orthopyroxene 0.7 ORTHOPYROXENE h0.6 MG/MG+FE "0.9 -0 8 CLINOPYROXENE MG/MG+FE 0.7 PLAGIOCL ASE -90 AN (mol®) 60 -2.0 TI02 (wt«) 1 .0 50 SI02 (wt«) h 40 30 -1.0 MG0/MG0+FE0 toi (wt55) -0.5 ilmenite BJ biotite amphibole cpx pUgioclase olivine opx chrome 0 spinel 3emple numbers 1 km Fig. 3. Variation in chemical composition of minerals and rocks. Cross-section AB is shown in Fig. 2. The predominant rock type at Raisduoddar- tinuous and vary from centimeters to several Halti is distinctly layered troctolite, the layering meters in thickness. In places, magmatic linea- being due to the modal variation in olivine and tion is well developed. Granulation and the in- plagioclase abundances. The layers are discon- tense kelyphytic reaction between olivine and 20 Pekka Sipilä plagioclase have obliterated cumulus textures in ments Cr, Co and Ni. The main components were many places (Sipilä 1988). The accessories are analysed by XRF and AAS at the Department chrome spinel and augite. of Geology, University of Turku and the trace With the increase in clinopyroxene abundance, elements by XRF at the Technical Research the troctolite grades into olivine gabbro. Centre of Finland in Espoo. Mineral analyses Stratigraphically in the highest position, the oli- were made at the laboratory of Material Sciences, vine gabbro is usually massive in texture, al- Department of Physical Sciences, University of though in the westernmost portions of the mas- Turku using energy dispersive (EDS) technique.
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