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The contents of rare earth elements (REE) have been determined in f i v e Svecofennian metamorphosed volcanic rocks and are compared wi th those found in the Karelian zone and Recent tholeiites. In the Svecofennian zone, R E E contents in low silica volcanic rocks are like those in the calc-alkalic series and in continental tholeiites, whereas in the Karel ian zone they are like those in island arc tholeiites. It is argued that in the Karel ian zone, where magmatism initiated, tholeiitic basalts originated in the upper mantle; as shown in normalized fo rm, the light R E E are depleted, reflecting the distribution pattern found, f o r example, in rocks of pyrrol i te composition. A s orogeny advanced, more and more crustal material — basement and geosynclinal sediments — was recycled and the R E E distribution pattern changed to that found in the synorogenic calc — alkalic rocks ( 1 . 8—1.95 Ga), ref lecting the R E E pattern present in sediments. The chemical composi t ion of the magmatic rock series fo rmed during Svecokarelian fo ld ing is quite like that observed in island arc systems.

Rare Earth Elements in Middle Precambrian Volcanic Rocks of Finland, with a Discussion of the Origin of the Rocks