A FLOW-FOLIATED IGNIMBRITE RELATED TO THE ALAND RAPAKIVI GRANITE IN SW FINLAND

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.