Bulletin of the Geological Society of Finland (Jun 1991)
Mafic and ultramafic igneous rocks of the Raisduoddar-Halti area in the Finnish-Norwegian Caledonides. Petrography, mineralogy and geochemistry
Abstract
The mafic and ultramafic rocks in the Raisduoddar-Halti area constitute a shallow 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 Ridnitsohkka gabbro sills in the east and dunite-troctolite-olivine gabbro cumulates at Raisduoddar- Halti. They are syngenetic and belong to the same nappe. The lower contact 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 intruded by coarse grained gabbro pegmatoids. The same pegmatoid magma gave rise to the metasomatic olivine gabbro with augite porphyroblasts between the cumulate massif and the sill area. Before last nappe movements the rocks of the cumulate massif were partly amphibolitized across the lithologic contacts and magmatic 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.
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