Comparison of the evolution of magmas in the Precambrian of southwestern Finland with that in the Paleozoic and Mesozoic of the northern Sierra Nevada brings out features that may clarify the origin of potassium-poor silicic magmas. In the northern Sierra Nevada, Paleozoic sodarhyolitic effusive rocks and associated trondhjemite represent silicic differentiates of andesitic magmas formed near a Benioff zone. These potassium-poor magmas were formed early, before thickening of the crust, and were followed by basaltic and rhyolitic magmas with normal potassium content. In southwestern Finland, where 70 percent of the area is covered by silicic and intermediate plutonic rocks, the early synkinematic intrusive masses are trondhjemitic and the later ones are granitic with eutectic ratios of quartz, plagioclase, and potassium feldspar. The latest granites are exceptionally rich in potassium feldspar. The oldest rocks, cordierite-garnet-sillimanite gneisses and interbedded metavolcanic rocks, are folded on gently plunging axes that steepen diapirically around large late-kinematic plutonic masses. Trondhjemite occurs as thin sheetlike masses parallel to the folded bedding and could not have traveled far without losing its initial heat. Therefore it seems that the trondhjemitic magma was formed at shallow depths. By analogy with the shallow depth of early magma generation in the northern Sierra Nevada, it is suggested that the trondhjemitic magmas in Finland formed at mantle depths near a Benioff zone or at the base of the early thin crust at pressures where phlogopite or biotite was stable. Later, after thickening of the crust above, potassium from the biotite was released, making the late kinematic magmas rich in potassium. A plate-tectonic model of an island-arc environment explains the coeval age of the Svecofennian and "Karelian" foldbelts and the increase of potassium with decreasing age in the extrusive and intrusive magmas.
