MAGMA MIXING, THE PETROGENETIC LINK BETWEEN ANORTHOSITIC SUITES AND RAPAKIVI GRANITES, ALAND, SW FINLAND

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.