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A flow‐foliated ignimbrite related to the Åland rapakivi granite in SW Finland
Author(s) -
Eklund Olav,
Shebanov Alexey,
Fröjdö Soren,
Ylikyyny Kari,
Andersson Ulf B.
Publication year - 1996
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1111/j.1365-3121.1996.tb00784.x
Subject(s) - felsic , geology , mafic , geochemistry , batholith , magma chamber , magma , phenocryst , petrology , volcanic rock , volcano , tectonics , seismology
A flow‐foliated felsic ignimbrite constitutes the uppermost lithological unit of the 1.58 Gyr anorogenic magmatic rocks in SW Finland. The ignimbrite is derived from an explosive eruption of hot (≅ 950 °C) phenocryst‐bearing A‐type (rapakivi‐type granite magma. The ignimbrite is close in composition to subvolcanic rapakivi granites that occur in the margins of the kand rapakivi batholith. The subvolcanic granites crystallized under a pressure of ≅ 1 kbar and at temperatures of about 650–700 °C. However, both major and rare earth elements show that the ignimbrite‐ forming magma was more fractionated than the magma forming the subvolcanic varieties. Supported by evidence of mafic‐felsic magma mingling, it is suggested that injection of hot mafic magma into a shallow magma chamber produced the high temperature of the ignimbrite‐forming magma. This injection increased the magmatic and the volatile pressure that caused the eruption of the dry felsic magma.