SALONSAARI, PEKKA T. 1995. Hybridization in the subvolcanic Jaala-Iitti complex and its petrogenetic relation to rapakivi granites and associated mafic rocks of southeastern Finland. Bulletin of the Geological of Finland 67, Pan lb, 104 pages, 56 figures, 7 tables, and 2 appendices. The 1630 Ma Jaala-Iitti complex is an example of bimodal rapakivi granite magmatism in which the interaction of granite and diabase magmas have led locally to hybridization. The dyke-like complex is situated at the northwestern margin of the Wiborg rapakivi batholith in southeastern Finland, cutting both the Proterozoic Svecofennian metamorphic crust and the Wiborg batholith. The complex consists mainly of non-hybridized compositionally homogeneous granites, i.e., hornblende granite and hornblende-quartz-feldspar porphyry which represent the felsic end-member (ca. 68 wt% Si0 2) of the complex. The mafic end-member (ca. 51 wt% Si0 2) is present as globules of disaggregated Fe-rich tholeiitic magma forming magmatic mafic enclaves (MMEs) and composite MMEs. Commonly MMEs and large (up to 2 metres in diameter) pillow-like MMEs show magma mixing and mingling characteristics. Hybrid rocks in the complex are monzogranitic and are characterized by (a) quartz grains and quartz aggregates (partially melted xenoliths) mantled by amphibole rims (ocellar texture) with occasional augite, (b) alkali feldspar megacrysts mantled by a micrographic plagioclase-quartz intergrowth, and (c) alkali feldspar megacrysts mantled by a plagioclase shell with occasional amphibole inclusions. These textures are also found in hybrid MMEs and especially in pillow-like MMEs. Alkali feldspar and quartz megacrysts in hybrid rocks are derived from partially crystallized felsic magma and from disaggregated rapakivi granite and granitoid xenoliths whereas in the MMEs the xenocrysts are solely derived from partially crystallized felsic magma. Disaggregation of the mafic magma to form MMEs and equilibration with the host have produced micro-enclaves and recrystallized borders of MMEs. Disaggregation of the mafic magma has also produced needle-like apatite crystals incorporated from the mafic magma into the hybrid magma; acicular apatite is typical in MMEs and pillow-like MMEs and is indicative of rapid crystallization. The mass-fraction of mafic magma (X m) in the hybrid rock is up to 0.3. The X m in hybrid MMEs and pillow-like MMEs varies from 0.4 to 0.9. The temperature difference between the two magmas probably caused convection that spread the disaggregated mafic magma throughout the postulated layered magma chamber. The injected mafic magma simultaneously mingled and mixed with the felsic magma which was superheated, assisting the chemical exchange. The interaction of hybrid MME magma with the hybrid magmas was possibly made more effective by diffusion of fluorine from the felsic magma into the mafic magma. The composition of minerals indicates intensive interaction of the mafic magma and the felsic host, with the temperature of equilibrium being in the range of 900° t...

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