Porphyritic pyroxene-bearing granitolds - a strongly weathered rock group in central Finland

Abstract

Intense preglacial weathering is associated with porphyritic pyroxene­ bearing quartz syenites, quartz monzonites, quartz monzodiorites and monzogranites, which form synorogenic stocks and batholiths in central Finland. The saprolite layers, which are 0.5‑2.0 m thick, or sometimes even more, cover thousands of square metres in places.

Often occurring on the lee side of the hills, the saprolite beds are usually covered with till. Strongest weathering always follows the sheeting and

fracturing of the rocks. The thicker saprolite layers contain roundish core stones. Saprolite is coarse‑grained resembling gravel in grain size, although the strongly weathered types contain prominent amounts of finer fractions.

The abundance of clay minerals is low. Two stages have been recognized in the weathering process. The first stage is characterized by granular disintegration followed by the subsequent alteration of biotite into vermiculite, and leaching of K, Na and Mg. The conspicuous increase in iron in the saprolite is probably due to precipitation of the element from groundwater. During the second weathering stage plagioclase is altered into kaolinite, and the breakdown of dark minerals promotes leaching of Ca, Na, Fe and Mg. The residue has been slightly enriched in K‑feldspar, quartz and clay minerals. Si and A1 have not dis­solved during weathering.