A flow-foliated felsic ignimbrite constitutes the uppermost lithologic
al unit of the 1.58 Gyr anorogenic magmatic rocks in SW Finland. The i
gnimbrite is derived from an explosive eruption of hot (approximate to
950 degrees C) phenocryst-bearing A-type (rapakivi-type granite magma
. The ignimbrite is close in composition to subvolcanic rapakivi grani
tes that occur in the margins of the Aland rapakivi batholith. The sub
volcanic granites crystallized under a pressure of approximate to 1 kb
ar and at temperatures of about 650-700 degrees C. However, both major
and rare earth elements show that the ignimbrite-forming magma was mo
re fractionated than the magma forming the subvolcanic varieties. Supp
orted by evidence of mafic-felsic magma mingling, it is suggested that
injection of hot mafic magma into a shallow magma chamber produced th
e high temperature of the ignimbrite-forming magma. This injection inc
reased the magmatic and the volatile pressure that caused the eruption
of the dry felsic magma.