O. Eklund et al., MAGMA MIXING, THE PETROGENETIC LINK BETWEEN ANORTHOSITIC SUITES AND RAPAKIVI GRANITES, ALAND, SW FINLAND, Mineralogy and petrology, 50(1-3), 1994, pp. 3-19
The Aland rapakivi batholith consists of several granites that differ
texturally and mineralogically from quartz-porphyritic varieties to ra
pakivi varieties with K-feldspar ovoids (wiborgites and pyterlites) an
d aplitic granites. Closely associated with the batholith there is a m
afic magmatic series of dolerite dykes, norites, anorthosites and monz
odiorites. The earliest major intrusive phase of the Aland rapakivi ba
tholith consists of quartz-porphyritic hornblende rapakivi. This rock
contains small amoeboidal mafic enclaves, labradorite megacrysts, quar
tz ocelli, amphibole-mantled xenoliths and irregular clots of granophy
ric granite. These disequilibrium features are products of mixing betw
een basaltic and granitic magmas. Geochemical modelling indicates that
the quartz-porphyritic hornblende rapakivi is a mixture of 15% hi-Fe
monzodiorite (mafic end-member) and 85% quartz-feldspar porphyry (fels
ic end-member). The monzodiorite is derived from a norite-anorthosite-
monzodiorite series. The quartz-feldspar porphyry is produced by parti
al melting of the country rock caused by intrusions of hot basic magma
. Structural, textural and geochemical features suggest that magma mix
ing was an important petrogenetic process in the formation of the earl
iest rapakivi granite intrusions in the Aland rapakivi batholith. Petr
ographic evidence of magma mixing can also be found in the major intru
sion of the batholith, the wiborgite rapakivi granites. Chemically the
mixing is difficult to specify in these rocks because of a high propo
rtion of felsic component. Zircon and apatite fractionation trends, ho
wever, indicate that the wiborgite rapakivis also contain components f
rom a mixed source.